Why Is It A Colour Wheel & Not A Line?

Summary

The Rest Is Science – “Why Is It A Colour Wheel & Not A Line?”
Podcast: The Rest Is Science
Hosts: Professor Hannah Fry & Michael Stevens (Vsauce)
Date: August 5, 2026

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Episode Overview

In this vibrant and interdisciplinary episode, Professor Hannah Fry and Michael Stevens explore the science and perception of color, dissecting the fundamental question: why do artists use a color wheel when physics describes light as a spectrum or a line? The episode bounces between Michael’s latest color obsession sparked by a mammoth box of Crayola crayons, the physics of the electromagnetic spectrum, biology of vision, color theory in art and practical applications spanning from make-up to technology. The duo field listener questions on fluid dynamics, "wetter" water, and the cybersecurity of satellites, infusing fun, curiosity, and memorable real-world experiments.

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Key Discussion Points & Insights

1. Michael’s Crayon Odyssey & The Color Wheel Paradox

• Michael’s New Obsession: Michael shares his meticulous process of organizing 152 Crayola crayons, sparking a deep dive into color theory. (00:10–02:45)
  • He physically swatches each color, noting the difference between shades like scarlet and red, and realizes the challenge isn’t linear—it’s multidimensional.
  • “I gotta arrange these. That means I’ve gotta learn color theory. And that began what has now consumed my life.” — Michael Stevens (01:00)
• Spectrum vs. Wheel: Why do scientists use a linear spectrum for light, while artists use a wheel?
  • The electromagnetic spectrum is a strict line (from long radio waves to short gamma rays), while the color wheel maps how we mix and perceive colors.
  • “It’s a line. It doesn’t connect back on itself... Why is it a wheel?” — Michael Stevens (05:50)
• Primaries Muddle: Priamry colors differ by discipline (physics: red, blue, green; art: red, yellow, blue).
  • “It depends what artist you ask... There are no primary colors. It’s all made up. You can choose any group...” — Michael Stevens (07:01)
• Purple/Magenta: Not in the Spectrum:
  • Human vision has only three types of cones (short/blue, medium/green, long/red). There’s no “purple” wavelength; purple/magenta exist only as a psychological blend of red and blue light, not a physical spectral color.
  • “The reason we put color in a wheel... is that purple does not exist.” — Michael Stevens (07:28)
  • “It is the color purple or magenta that is purely a psychological phenomenon, not a spectral color.” — Michael Stevens (09:31)

Timestamps:

• Explanation of color spectrum vs. color wheel (04:52–09:37)
• The mystery of purple & magenta (07:28–09:37)

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2. Perception, Technology, and Real-World Color

• Why Crayon Colors on Camera Don’t Match Reality:
  • Some Crayola crayon colors, especially neons like Outrageous Orange, Neon Carrot, Atomic Tangerine, cannot be captured by cameras or screens because they’re fluorescent—these convert invisible ultraviolet light into bright visible colors.
  • “You cannot photograph this the way it actually appears because it’s fluorescent... reflecting more light than is landing on it.” — Michael Stevens (10:30–11:46)
  • Demonstrates live with examples of how vibrant colors look muted or altered on digital displays.
• Camera Sensors & Infrared:
  • Camera sensors detect infrared, turning the invisible visible by rendering it as purple on screens (e.g., TV remotes, FaceID in night vision).
  • “Phone cameras and screens can also pick up infrared colors that we cannot see.” — Michael Stevens (16:42–17:34)
  • “You can see it’s flashing onto their face... that in real life you do not ever see at all.” — Hannah Fry (17:34–18:24)
• Why Browns Can’t Be Neon: Neon means “brighter than ambient light”—brown, by definition, is a dull (dimmed) orange; the two are opposites.
  • “Brown is not only not neon, it’s like the opposite. It’s knee off.” — Michael Stevens (19:16)

Timestamps:

• Fluorescent crayon colors & camera limitations (10:15–12:45)
• Infrared experiments with cameras (16:42–18:24)
• Neon vs. brown (19:16–19:46)

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3. Color Theory in Everyday Life: Makeup and Visual Tricks

• Makeup Artists as Color Theory Experts:
  • Hannah explains how makeup uses complementary colors to neutralize skin imperfections (opposite color blending).
  • Viral “color theory witches” on TikTok/Instagram mix vivid colors to create perfect skin tones through blending.
  • “The whole point of makeup is that you are changing lightness and darkness... you’re essentially changing... to give the illusion that your face is doing something different...” — Hannah Fry (19:16–20:29)
  • “They will chuck on... vivid green, then purple, then yellow, and... it will perfectly match the color of their face.” — Hannah Fry (20:44)

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4. Listener Questions

A. Fluid Dynamics: The Tea Leaf Paradox

• Question: Why do tea leaves collect in the center when you stir tea?
  • Hannah (fluid dynamics expert!) explains the boundary layer: friction at the cup’s bottom slows down fluid, creating secondary flow that sends leaves to the center, not the edges. It’s a classic question, first analyzed in the 1800s, with parallels to riverbank erosion.
  • “It’s called the tea leaf paradox... the key reason is because the top of your cup is not the same as the bottom... friction is causing this great big kerfuffle.” — Hannah Fry (24:58–26:43)
  • Turbulent stirring (not circular) dissolves sugar faster—do it “erratically”!
  • “I stir tea like this... do an erratic shape with your teaspoon. You look completely insane... but it’s much quicker.” — Hannah Fry (30:04–30:21)
  • “Boundary layer and fluid dynamics... was quite literally what my PhD was doing.” — Hannah Fry (26:43)

B. Can Water Be “Wetter”? (Surfactants & Firefighting)

• Question: Can water become “wetter”? Why does water sometimes “feel more wet”?
  • In firefighting, “wetter water” (water with reduced surface tension via additives/surfactants) penetrates burning material better than plain water—making it “wetter” for that purpose.
  • “They had to come up with ways to make water wetter... Basically, soaps. And these make the water wetter.” — Michael Stevens (33:34)
  • “Wetter water” (e.g., firefighting products like ‘Wetter Water’) exists!
  • “In your lungs... you’ve got this wetter water... so you can actually breathe.” — Hannah Fry (35:49)
  • Philosophical split: “wet” as “sticks to” vs. “penetrates,” with no one simple answer.

C. Hacking Spacecraft & Satellite Security

• Question: Can old or modern satellites be hacked? How secure are their communications?
  • Older probes (e.g., Voyager 2) lack modern encryption but are very hard to reach due to distance and specialized antennas needed.
  • Modern satellites use advanced encryption—but have still been vulnerable; e.g., 2022 attack on ViaSat disrupted Ukrainian military and wind turbines in Germany:
    • “Attackers managed to reach the management network of ViaSat's Kasat system…pushed a virus…bricked them.” — Hannah Fry (42:17–43:37)
  • Quantum computing could one day threaten even good encryption.
  • “You have to hope that encryption keeps up with the computing.” — Michael Stevens (44:26)

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Notable Quotes & Memorable Moments

• The core of the wheel vs. spectrum paradox:
  “The reason we put color in a wheel, even though physically it lies on a line, is that purple does not exist.” — Michael Stevens (07:28)
• Color as a mind game:
  “…It’s the color purple or magenta that is purely a psychological phenomenon, not a spectral color.” — Michael Stevens (09:31)
• On fluorescent colors’ unphotographable reality:
  “You cannot photograph this the way it actually appears because it’s fluorescent... reflecting more light than is landing on it.” — Michael Stevens (11:46)
• Makeup artistry meets science:
  “Every morning... because I know the color theory, I get a tiny bit of orange... and I paint a bit of orange on top of the blue because they are opposite on the color wheel.” — Hannah Fry (19:56)
• On water and its “wetness”:
  “They had to come up with ways to make water wetter... Basically, soaps. And these make the water wetter.” — Michael Stevens (34:37)
• On ‘wetter’ water and the lungs:
  “You’ve got this wetter water in your lungs to make it so that you can actually breathe.” — Hannah Fry (35:50)

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Additional Highlights

• Hands-on science: Michael’s DIY crayon palette sparks tangible discussion of real vs. digital color.
• Accessible experiments: Listeners are encouraged to try seeing infrared with their smartphones.
• Interdisciplinary joy: From crayons to makeup, satellites to firefighting, the episode celebrates curiosity dissolving boundaries between science and the everyday.

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Segment Timestamps

| Segment | Topic | Time (MM:SS) | |---|---|---| | Opening | Crayon obsession, color theory intro | 00:04–02:45 | | Sponsor/Ads skipped | 02:45–04:52 | | Science of spectrum vs. color wheel | Core episode discussion | 04:52–09:37 | | How purple "doesn't exist" | Color perception | 07:28–09:37 | | Cameras, screens, and neon colors | Technology & perception | 10:15–12:45 | | Infrared light and what phones can see | Fun experiments | 16:42–18:24 | | Makeup & color theory | Real-world color use | 19:16–21:03 | | Listener Q#1 | Fluid dynamics—tea leaf paradox | 24:26–30:21 | | Listener Q#2 | Can water be wetter? | 33:00–37:54 | | Listener Q#3 | Satellite hacking | 39:15–44:26 |

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Summary

This episode is a technicolor exploration of how our tools, brains, and traditions shape the experience of color. With stories from art, science, and daily life, Hannah and Michael reveal the quirks of our senses, the cleverness of engineers, and the endless joy of questioning the world. Whether pondering why some colors can’t be captured on camera, or how surfactants beat fire, or how satellites dodge hackers, the show proves the most basic phenomena — color, water, light — are endlessly fascinating and subtly strange.

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A dazzling, brain-tingling listen for anyone curious about how we see and shape our world!

Transcript

[Hannah Fry]

Welcome to the Rest Is Science. I'm Hannah Fry.

[Michael Stevens]

And I'm Michael Stevens.

[Hannah Fry]

And this is Field Notes, our weekly expedition into the mind, this week of Michael Stevens.

[Michael Stevens]

All right, so what's going on in my mind? I'll tell you. I gotta apologize. I've gotten really into something brand new once again. This time it's Crayola crayons. So I'm still a mechanical pencil guy, don't get me wrong.

[Hannah Fry]

Sure.

[Michael Stevens]

But I. I visited my mom a few days ago, and she had this box of 152 crayons she'd bought for this coffee shop. Turns out they didn't need them. She was gonna return them. And I'm like, well, I'll take them. And they don't come ordered by any sort of color theory that exists in human knowledge. Right. It looks random. It isn't. But it's just based on the skew of the products. Like they just shoved together the eight pack, the 16 pack, the 24 pack and that. So it's a mess. And I'm like, I gotta arrange these.

[Hannah Fry]

But it's 152 distinct colors.

[Michael Stevens]

One hundred and fifty two distinct colors. And I'm like, I gotta arrange these. That means I've gotta learn color theory. And that began what has now consumed my life. I've had to get special paper to do swatches.

[Hannah Fry]

Oh, my gosh, this is amazing.

[Michael Stevens]

So, because I'm looking at the wax going, okay, but what is the difference between scarlet and red?

[Hannah Fry]

Okay, for the purpose of those of you who are joining us in audio only, first of all, what are you doing? Secondly, you are missing Michael holding up what looks like a one of those fancy spiral bound artist notebooks. Except instead of it being some sketch of a building or a face, he has laid out every color crayon that he has in his pack, and each one he has labeled with the name of that crayon. He's also done a gradient from very dark, extremely heavy crayon all the way through to very light. But what is interesting about this also that I. That I already notice is that this is not a linear progression of colors. They appear in a grid. I think there's something going on here. And genuinely, I say this without any humor whatsoever. I am thrilled that this is the subject today.

[Michael Stevens]

I should show you that I've got piles of different crayons. I had to buy different packs because some of them don't give you exactly the. You can't finish the spectrum unless you get the colors of kindness pack and the 96 has dandelion in it, but the 152 doesn't. Anyway, I've learned so much, and so today I want to show off a couple of cool things about color, but especially I want to show you some things that. That I can't show you.

[Hannah Fry]

Oh, I'm so excited. This episode is brought to you by Cancer Research uk.

[Michael Stevens]

Our bodies are incredible machines, whirring away, making more and more DNA to build the proteins that keep us alive.

[Hannah Fry]

In fact, in the last minute, your body has made over 200 million new cells and enough DNA to stretch to the moon and back to the moon and back.

[Michael Stevens]

That's so much DNA that if you compared it to the size of the cell it fits into, that would be like squeezing the London underground into a suitcase.

[Hannah Fry]

By the age of 50, you have copied almost 6 trillion miles of DNA. But every time that your body copies DNA, it risks making mistakes. And over time, those mistakes can accumulate and that collection of errors can lead to cancer.

[Michael Stevens]

But incredibly, Cancer Research UK scientists can spot these errors. And by finding them, They've helped double UK cancer survival over the last 50 years and are driving even more discoveries that could tackle over 200 types of cancer.

[Hannah Fry]

For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org REST ISscience

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[Michael Stevens]

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[Hannah Fry]

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[Michael Stevens]

Okay, so we'll start with the biggest mystery. And this struck me as I was driving around yesterday. Literally. This is how fresh this whole thing is. I was like, wait a second. The electromagnetic spectrum. Okay. Which is how we lay out all the different energies light can have. Makes sense to me. I get it. You've got radio waves. You make them more energetic.

[Hannah Fry]

Oh, whoops.

[Michael Stevens]

Now they're microwaves. Eventually you get into visible light that our eyes can see. And if you crank up the energies. There you go from red light to orange, yellow, green, blue, violet, and then ultraviolet all the way up to X rays, gamma rays. It's a line. Okay.

[Hannah Fry]

A linear scale. Yeah.

[Michael Stevens]

You either have more energy or you have a longer wavelength than, say, 700 nanometers or less. It's one way or the other along one line. And yet artists are always telling us about color wheels. What the heck?

[Hannah Fry]

Come on. You guys.

[Michael Stevens]

You guys, it's. It's a line. It doesn't connect back on itself. It's not like when you reach violet, you add more energy and it's red again. Why is it a wheel? I'm holding up a color wheel, by the way. This is just a classic artist's coloring wheel. You might see this in, like, elementary school, even where the teacher wants to talk about, you know, how the colors mix and how they change where they fall on the spectrum. And this helped me sort my crayons really well because I just did not know when I looked at a red. Does that red have more orange in it or yellow? So this allowed me to, like, add more red to yellow, orange and be like, oh, to hold the crayon up, and then if it was a tone or a shade or a tint, I can use the back to kind of determine where it should go. I can sort the grays, you know. Anyway, why do artists use a color wheel but scientists use a color spectrum? That's a line, right?

[Hannah Fry]

This is genuinely. This is something I've always wondered in addition, which is sort of the same thing, you know, if you ask a physicist what the primary colors are, they will tell you that it is red, blue, and green. But if you ask an artist, they'll tell you it's red, yellow, and blue. And I never understood what, what, how, why.

[Michael Stevens]

Look, it depends what artist you ask. I think if you ask an artist who has also become obsessed with this, they will say there are no primary colors. It's all made up. You can choose any group of colors to be your primaries. It might not be as helpful, but it's not like the universe at the Big Bang was like, red, blue, green. Oh, my gosh. Look at all the other stuff that you can make out of this. No, there's. That's not how it works.

[Hannah Fry]

Okay.

[Michael Stevens]

The reason we put color in a wheel, even though physically it lies on a line, is that purple does not exist.

[Hannah Fry]

Excuse me.

[Michael Stevens]

Yeah, here's the problem. So you've got a spectrum of color that means a spectrum of energies light can have that causes our brain to experience different colors, from red all the way up to the really small wavelength and high energy blues and violets. Now, violet is a spectral color. That means it's created from light. It's a real thing that has physical properties. And our eyes, the human eye is only sensitive to basically the short, the medium, and the longs. Okay? If your eye receives short and medium wavelengths, then the perception in your brain is that it must be somewhere in between. So maybe it's orange. Okay? If you get a bunch of middles and a bunch of longs, then your brain goes, well, we'll give you the experience of blue, okay, or maybe like a blue green.

[Hannah Fry]

Because you only have the three cones, right? That in your eyes you have the three cones which are like, is there any red there? Yes or no? I mean, and how much yes and how much? No. Is there any blue there? Is there any green there? You haven't got an orange cone. You're right, there isn't one. There isn't a purple cone.

[Michael Stevens]

But we can see orange because our brains are able to process, hey, the shorts and the mediums are firing. So let's call it orange. And by call, I mean it creates the experience of orange on that object in the world for you. But here's the big question. What happens when your eye is receiving both short and long? It doesn't give you middle as a perception, because that would be triggering the middle, guys. It'd be green. So instead, our brains have created a color that does not physically exist, and it is the color purple or magenta that is purely a psychological phenomenon, not a spectral color.

[Hannah Fry]

Wow. Never thought about this. This is so good. This is so good.

[Michael Stevens]

And so that's why we're able to do, starting at red, increase the wavelength, until all of a sudden, boom, you're at violet. And then it goes back, because in between the violet and the red, we have a combo that we experience as a mental construct, and that is purple or magenta type colors. Okay, so then I'm start. I'm chronicling my crayons, I'm taking photos. Uh oh. Phone camera sensors cannot recreate every color that Crayola makes. Go on, let me show you a color. This one.

[Hannah Fry]

Okay. This is like a. It's orange, but it's like a reddy orange, sort of like the color of a tomato as it becomes ripe. But before it goes into that deep red.

[Michael Stevens]

Wrong. This color does not look anything like what you just described to me. Seeing it with my own eyeballs. What does it look like you're seeing? It only as the camera and your computer screen can show it to you. This is a color that Crayola calls Outrageous orange. And it is brighter than safety orange. It's like a high vis, shocking, hurts your eyes orange. And yet on camera, it looks like, oh, that's a tomato. Or, you know, it looks almost creamy. It's almost like a salmon to me on my screen. But in real life, you. Yeah, this thing is Outrageous Orange. That's the perfect name for it. And you cannot photograph this the way it actually appears because it's fluorescent. That's the problem. The way this works, the reason this color is so bright in real life. And if you ever stumble upon a box of crayons, just open it up, pull out an outrageous orange, and you'll see what I mean. Basically, Crayola has added special optical brightener chemicals to this that take in ultraviolet light, which is coming in from all kinds of sources, especially the sun. And those chemicals convert that invisible ultraviolet light down into visible light that we can see that's re emitted. And so the crayon is brighter than it should be. It is brighter than any normal thing in the world because it's actually producing. It's reflecting more light than is landing on it that we can see.

[Hannah Fry]

Hold on, hold on. Can I see some of it on the page? Can you draw some on the page?

[Michael Stevens]

I can show you this corner up here. I'm going to show my swatches again. So Outrageous orange is right there.

[Hannah Fry]

Right?

[Michael Stevens]

And it looks a lot more like a carrot. It's a little bit more brown. There are three colors here on my swatches that really do not come across on screen. The three are Outrageous Orange, Neon carrot, and what's this one called? Atomic Tangerine. So these three look almost more like an earth tone on a screen. Yeah. Outrageous orange does look a little lighter than natural.

[Hannah Fry]

It still looks ready. It doesn't look like. It doesn't look like hazard orange at all. It just looks like a sort of a ready ready orange smudge.

[Michael Stevens]

I know. And so this is what's sort of disappointing about this episode. So if you're listening only, it doesn't matter. The people watching aren't seeing anything different because the camera cannot capture this. Now, neon carat is really interesting because neon carat is another neon fluorescent color that has optical brighteners in it that make its color brighter than physically possible without light being literally changed in energy levels. But neon carrot on screen looks almost exactly like regular old orange. So can you tell which one? They clearly are different, but which One of these is the fluorescent neon carrot. And which one is just standard orange?

[Hannah Fry]

Okay. So you're holding two up to the camera. I mean, honestly, they look almost identical. They both look sort of traffic cone orange. One of them is slightly redder than the other. I'm gonna go for the. I think the slightly red redder one is.

[Michael Stevens]

The.

[Hannah Fry]

Is normal orange.

[Michael Stevens]

Okay. This one. This one, you're wrong. This one is neon carat. Okay. And this one almost looked brighter. This is just the regular orange that you would get in an eight pack, you know, at a restaurant. This is just regular orange. But to me, actually, in front of these crayons, this one that you thought was regular orange, this one is bright. This is a, like, high vis. You would never miss it. It's. You could spot it across the room kind of orange. Side note, because all you guys are going to be wondering, what are Michael's favorite Crayola colors? These are them. And they. They do appear on screen exactly the way they appear in real life.

[Hannah Fry]

Okay, that was not the colors I thought you were going to go for. For those of you who are listening, Michael is holding up two pens. These are, you know, apparently his favorites. One of them is a gray brown. The other one is a red brown. Yeah, I look. They're muted tones.

[Michael Stevens]

They're not muted tones. Their loudness is why I love them. I don't know why. What is going on. But the maroon crayon is like paint. It gets so thick, you put a line on a sheet of paper, you'd see it from a mile away. I don't get it. I can show you this on the swatches.

[Hannah Fry]

Yes, please.

[Michael Stevens]

Okay. So maroon is this one that's screaming at you right there. You see that?

[Hannah Fry]

Oh, yeah, yeah. There is. A lot has come down there.

[Michael Stevens]

A lot comes down. It is such a pleasure to draw with. It is like drawing with blood. I don't know. It can't just be the pigment. Unless the pigment is chemically changing the wax a little bit. But it's night and day. There's no other crayon like it. The browner one, this one, this is called shadow.

[Hannah Fry]

Right.

[Michael Stevens]

And it's pretty fun to draw with, but it is so confusing. Try to draw with a shadow. Crayola. And it's not a mix of brown and green. It's. It's sometimes brown, sometimes green, depending on where you decide to look. It is so strange. I can't wrap my head around it. I've been thinking about it every day.

[Hannah Fry]

What depending on the light. That's hitting it?

[Michael Stevens]

No, it depends on, like, maybe it depends on the way the wax is structurally on the surface. Because I'll show you the swatch. Okay, here's shadow. Shadow is this one.

[Hannah Fry]

It does look green. It looks very green on camera.

[Michael Stevens]

It looks pretty green on camera, but it looks like, you know, where it's thicker, it becomes browner. But sometimes for no reason whatsoever, it looks more green in areas than others.

[Hannah Fry]

Do you think this is intentional part of the design of it?

[Michael Stevens]

I think someone there was, like, trying to make a greenish brown, and then they made this, and it scared them so much, they called it shadow.

[Hannah Fry]

The one grayola they all feared.

[Michael Stevens]

I've got one more thing to show you. This is. This is. This is something that I'm sure you've seen many times before. But phone cameras and screens can also pick up infrared colors that we cannot see. And they pick them up and then they convert them and display them as colors that we can see. The most easy way to see this is to use a television remote control or any kind of remote control that uses infrared light to communicate with a device. So if you look at the emitting light bulb on your remote and you push buttons, nothing happens. It looks like it's off. But point it at your phone and. And look at how it shows up on your phone screen. Or I'll point it at the camera right now and you guys can see.

[Hannah Fry]

We can see it.

[Michael Stevens]

There it is a little purple light. And that purple light is infrared light that the cameras can pick up on. They go, oh, there's light there. And then they render it as usually that kind of purplish color.

[Hannah Fry]

The version of that that I really enjoy is I have one of those doorbells, the smart doorbell things with the camera on it. And. And the one that I really enjoy is when it's nighttime and the camera switches on as somebody comes home. You know, whenever someone's coming home from they're getting a taxi home or whatever, and they open their phone. If you have face ID on your phone, you essentially does an infrared scan of your face. That's essentially how it's like a depth measurement of your face that it's doing. And when you watch somebody on something like a dual camera that's in night vision mode, where it is essentially doing the same thing as they open their phone. You can see it's flashing. It's flashing onto their face. This, like, very bright light flashes onto their face. That in real life, you do not ever see that at all.

[Michael Stevens]

Yes, that's another great experiment. To do it shows you how much of the world we don't see that's happening. But then when it comes to the fluorescent colors, the neon colors, there is a world that cameras still can't capture and show us that you have to see in real life. And I've got a whole short about neon colors and using how they work to create neon brown. What would neon brown look like? And as it turns out, the definition of brown is basically, it can't be neon. Neon means more light is coming out than is even available in the environment. But brown means that there's so little light coming out that it looks dim. Take an orange dim down the amount of light coming off of it, it's gonna look brown. So brown is not only not neon, it's like the opposite. It's knee off.

[Hannah Fry]

So neon and knee off together doesn't work. It doesn't. You gotta choose one. This is a light switch situation. Going back to your color wheel. Can I tell you the reason why I was so excited about this episode? Because there is one time where understanding color theory comes up on an almost daily basis for a lot of people, which is in makeup. Do you know about this? Okay, so I know that, like, makeup feels like quite a girly subject, but I'm just going to go with it for just a moment because I think this is absolutely. I personally think makeup is very, very interesting and exciting for these exact reasons. For these exact reasons that you are using the color wheel the entire time, and you are essentially changing. The whole point of makeup is that you are changing lightness and darkness on your face in order to correct or give the illusion that your face is doing something different than it actually is. Yeah, so. So, for instance, I have, like, these little blue. My skin is, like, basically translucent, so you can kind of see the veins just underneath my eyes. And so every morning, I wake up in the morning, and because I know the color theory, I get a tiny bit of orange. Like, a tiny, tiny little bit of orange. And I paint a bit of orange on top of the blue because they are opposite on the color wheel. And then sometimes I get little bit of red around here. So I get some green, and I put some. A little bit of green around here. But there are now these people on. On TikTok, on. On Instagram who call themselves color theory witches, and they. I don't know if you've ever seen this, Michael. It's so amazing. But they will start off with their completely clear face, and they will chuck on, like, some vivid green, and then Like a blodge of purple and then like a tiny little bit of yellow, and they'll just mix it in and it will perfectly match. Like, perfectly match the color of their face.

[Michael Stevens]

Yeah.

[Hannah Fry]

I find that so endlessly phenomenal and impressive.

[Michael Stevens]

It's impressive and it's impressive, but it's.

[Sponsor/Ad Voice]

It's.

[Michael Stevens]

It's reality.

[Hannah Fry]

Yeah.

[Michael Stevens]

Because your skin is not one color. It's not like a pure peach or whatever was like, put on you. Instead, all the colors are there, and if there's a lot of them, then you wind up with something that's very light in color because it's approaching whiteness. Right. White light containing all the wavelengths. And so you can combine green and orange and all these colors and make something that looks like it doesn't have any green in it at all to. To it to a naive perspective.

[Hannah Fry]

But it goes back to what you were saying earlier, that you've got, you know, your brain is only like, is it short, medium, or long? And how much short, medium, and long? You know, photons essentially are there that are hitting the back of my retina at this exact moment. That's literally it. That's all there is to it. Yeah, it was great.

[Michael Stevens]

Yeah. So I've got a lot more to learn about color. One of these days, I will have my crayons arranged in. It's gonna have to be a 3D shape because we've got three things going on. We've got how much gray, how much black, how much white. That's one spectrum, and then what the hue is. But I'm excited to show that off once I've got it.

[Hannah Fry]

Mm. Okay. This is good. I'm looking forward to this. Also, can I just say, what an absolute delight to get a little insight into the way you spend your time. Michael, this is really genuinely. I'm endlessly fascinated by you.

[Michael Stevens]

No, well, Hannah, I'm glad. I'm glad. I'm glad that I have someone to share it with. Not just you, but the listeners out there. Because otherwise I'm just sitting alone talking to myself at night, being like, ah, oh, okay. So is the Atomic Tangerine more red or orange? And then I try to, like, use my camera phone. Realized that doesn't work at all because it's a neon and boom podcast episode.

[Hannah Fry]

I'll tell you who else I'm endlessly fascinated by, Michael. Our listeners and their questions, which we will come to after the break.

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[Michael Stevens]

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[Hannah Fry]

You bet I am. Because the first one's about fluid dynamics. Michael.

[Michael Stevens]

I know, I know, I know. I love this one. So this comes from Patrick Farrell. He emailed this question in to us. By the way, if you want to do that, you can email [e-mail] Please do so just like Patrick did. Patrick asks, when I stir a cup of tea, the leaves collect in the center of the bottom rather than being flung to the edges, which to me feel since everything spinning should be pushed outward. I've read that Einstein once wrote about this and that the same effect explains why rivers erode their banks the way they do. What's actually going on in the cup?

[Hannah Fry]

Patrick, I don't believe for a second that you don't know the answer to this. But I think that you're, you're, you're here to amuse me because you've given yourself away in the question that you know so much about this already. But I love it and I'm very, very excited to get to talk about this. Okay, this is called the tea leaf paradox. Have you come across this before, Michael?

[Michael Stevens]

Yeah, I have, but I don't remember enough about it. So I'm just like Patrick. Well, no, I'm not like Patrick. Patrick probably knows the answer. He's like, well, Einstein wrote about it, but I think he wants to hear, he wants to hear how you explain it. And so do I.

[Hannah Fry]

Right? Okay, so he's absolutely right. If you stir a cup of tea, I mean, frankly, you should be straining the cup of tea before it ends up in your cup. But fine, if you end up with leaves in Your tea, and you stir it. Then the leaves, you expect they should get pushed out to the sides because the. The. The whole of the fluid is. Is sort of rotating in your cup. And you should expect they get kicked out to the sides, but they don't. They end up clustering in the middle. It doesn't make any sense. People have been like, what the hell is going on? For a really long time, since the 1850s, essentially. And the key thing, the key reason why this happens is because the top of your cup is not the same as the bottom of your cup. Because the bottom of your cup has a bottom, and the top of your cup is free to the air. The level of the fluid is free to the air. And so essentially what's going on here is friction is causing this great big kerfuffle, because as you have friction at the bottom, you get something called a boundary layer. This, by the way, was quite literally what my PhD was doing was in boundary layer and fluid dynamics. So you get, like, right next to the physical surface of your cup.

[Michael Stevens]

You.

[Hannah Fry]

It's completely jagged. It's like a mountain range if you zoom in. So the fluid cannot be moving at that point, and yet a little bit further up where your spoon is spinning around, it's going really fast. So what happens is you have this, like, this range of flow speed from zero to very fast at the bottom. And it's sort of like it drags along the bottom, right? It's sort of like it kind of. It goes slowest along the bottom, and then sort of slower, slower, slower, and then. And then kind of speeds up as it goes up, up towards the top. So all around the sides of the cup and all around the bottom, you've got this. This boundary layer, this friction, this kind of like dragging fluid. So what that ends up meaning is you have this, like, rotating fluid. It ends up. There's a secondary flow. So the dominant flow is round in a circle, but the secondary flow is that it kind of goes up, and then it has to go back down and round the sides. You know, your spinning tee is pushing outwards, but near the bottom of the cup, it's slower. So you end up with this hidden circular movement. And the thing is, is that once it gets down to the bottom, that is, like, so much more dominant than the spinning that's going on at the top. And so you end up with these leaves being pushed along by that. That secondary circular circulation. So this is. This. It is the same thing as goes on in rivers. Thank you so much, Patrick. But as Rivers go round corners, you end up with essentially the same thing. You have, like, fast flow on one side, then you have all of the, like, friction along the bottom. And so you end up with this secondary circulation that ends up dumping loads of sediment on the inside. Is that right? Yes, on the inside of the. The rivers. So you should be able to tell if I just take a photograph of a river, you should be able to tell based on the riverbank, which is the sort of inside and the outside bend. Even if you can't fully see the bend. If I show you a cross section of a river, you can, you can tell just from that, because of this effect, essentially. Anyway, fluid dynamics is just really great. And thank you for letting me have. Thank you for letting me have three or four minutes on it. It was worth all the training.

[Michael Stevens]

That's really cool. Yeah. I didn't know the answer was so sort of complicated.

[Hannah Fry]

Oh, okay. Maybe I explained it complicatedly.

[Michael Stevens]

No, no, no. What I mean is I thought that there was some like, oh, it's because of, you know, gravity. Why?

[Hannah Fry]

Is it because of friction? It's all because of friction. Sort of slows it down.

[Michael Stevens]

So when. When did we figure out the tea leaf paradox?

[Hannah Fry]

1857. It's. Yeah, 1850. Like, all of this stuff was going on was, you know, all of this boundary layer stuff was going on around this time, late 1800s, early 1900s. It's like this explosion in, like, how fluids flow. I should tell you one other thing, actually, when it comes to a cup of tea, if you are stirring your tea in a circle because you're like, oh, I want to get, you know, I really want to get this, this sugar stirred in really quickly. Terrible idea. Don't do it that way because actually the sugar will dissolve much better in a turbulent, turbulent fluid. Right.

[Michael Stevens]

Oh, okay, hold on. Let me, Let me, Let me guess. Because this is what I do when I need to, like, get salt to. To dissolve in pasta water as quickly as possible.

[Hannah Fry]

Yeah, don't do a circle. Don't do that.

[Michael Stevens]

Well, hold on. I'll do a circle this way and then I'll do a circle the other way really quickly. So it gets all, like, you know, turbulent and it feels like at least that fixes the problem of salt congregated in the bottom. Just like tea leaves.

[Hannah Fry]

Absolutely. You can also. I stir tea like this. Like, for people who are watching just listening, it's sort of an erratic. Just do an erratic shape with your teaspoon. You look completely insane.

[Michael Stevens]

Like you're just bouncing back and Forth off the walls randomly.

[Hannah Fry]

Yeah, it's like, oh, let's just, you know, wiggle it around a bit. It's much quicker. It's much quicker. Also, I should probably give up having sugar in my tea, but. But all the same, I'll tell you what, we should definitely do an episode on the effect of sugar on your brain. And because it's not good. It's not good.

[Michael Stevens]

Oh, I would love to. Yeah.

[Hannah Fry]

One for another time. But for now, sugar fans. Until then, you can. You can carry on enjoying the fluid dynamics of what's going on behind the scenes. Actually, I've just been reliably informed by Neil, who was watching the World Porridge Making Championships. That's a kind of caliber of human who works. Who works on our show. Michael.

[Michael Stevens]

Correct.

[Hannah Fry]

And he's told us that actually, when it comes to stirring porridge, you need to do it with your right hand because that keeps the devil out.

[Michael Stevens]

Oh, we forgot about the devil. That's right. Whoops. Continue stirring with your right hand. Probably clockwise, because, well, I mean, what do you want? You want. You want. You want Satan to, like, come bother you? Or do you want sugar that's not quite dissolved? Okay, your choice. All right, well, you know what? Our next question is actually a little bit about water as well. This one comes from our subreddit. The rest is science, where Jessxx says. So my boyfriend has this really weird theory proposing that water can be differently wet. He doesn't mean in terms of hardness. He means sometimes tap water is more wet than other days. Honestly, I just want to prove him wrong. But apparently no one has ever asked a question like this, so I can't find any studies on it. Can water get more wet? I loved that. I didn't know how to answer this, but I had to look into it, and I had to go all the way back to 1946.

[Hannah Fry]

Go on.

[Michael Stevens]

And as it turns out. Do you know where this is going?

[Hannah Fry]

No.

[Michael Stevens]

So I had to go all the way back to 1946 to fire engineering magazine. They wrote about this problem that firefighters have had for a long time, which is that sometimes water isn't wet enough.

[Sponsor/Ad Voice]

What?

[Michael Stevens]

So before I explain what that means, let's look at, like, what causes fire. Okay. Fire is a chemical reaction, and you need three, maybe four things to really have a fire start. You need the fuel, but you also need oxygen, and you also need heat.

[Hannah Fry]

Mm.

[Michael Stevens]

It's that third leg, the heat that water fixes. Water absorbs heat so well that it cools stuff down to the point at which the chemical reaction of combustion can no longer happen. The water isn't, like, washing away the elemental particles of fire. It's just cooling down the fuel to the point where it no longer combines with oxygen and combusts.

[Hannah Fry]

Mm.

[Michael Stevens]

Okay, here's the problem. A bale of cotton on fire is very hard to put out, because as soon as you put that water on and you need that water to seep into the cotton and cool it down, the cotton on the top, the surface, absorbs the water, and it stops the water from penetrating deep into the cotton to cool down the entire bale, especially in the middle. And this 1946 article is like, honestly, it's been shown that kerosene can put out a cotton bale fire faster than water.

[Hannah Fry]

No, because it slips through.

[Michael Stevens]

Because it slips through. Kerosene being this hydrocarbon, it just slips right into the cotton. And if you have enough kerosene, yeah, it's flammable, but if its temperature is low enough, it'll drop the temperature of the cotton all the way through. So quickly, combustion stops. The fire is out long before water could do it. And so they had to come up with ways to make water wetter to reduce its surface tension, to reduce the extent to which it sticks to things and itself. And they do this with all kinds of chemicals nowadays. They do it with, like, glycols and surfactants, obviously. Basically, soaps. And these make the water wetter. Yeah. And so let me send you, this is a hilarious link where you can buy, like, firefighting products. This is a product called Wetter Water, the water extender. And it is water that's more wet. It wets things. It clings to things and seeps through them faster than regular water.

[Hannah Fry]

So this here says, lowers surface tension, cools faster, super concentrated, safe for pumps and seals. Wow. I had no idea about this.

[Michael Stevens]

So as I dove into this, I saw a lot of people saying, oh, yeah, you know, I'm a firefighter, and we'll sometimes put soap into the water because it's going to soak through, especially fabric materials, absorbent materials, faster and cool them down faster. So, yeah, they use wetter water.

[Hannah Fry]

I am so blown away by this. I'm also thinking, aren't there surfactants in your lungs? Have I remembered that correctly?

[Sponsor/Ad Voice]

Ah.

[Michael Stevens]

To help. In what way? To help.

[Hannah Fry]

So the inside of your lungs is basically like a wet balloon. And surface tension would mean that they would collapse. They're sort of covered in this very thin layer of water. And because the water molecules are attracted to one another, it continues to pull this little sack into a very tight little droplet. So if your lungs just had pure water in there, then the surface tissue would be so strong that every time you breathed out, they would collapse flat and they wouldn't open again.

[Michael Stevens]

Right.

[Hannah Fry]

So you've got this wetter water in your lungs. In your lungs to make it so that you can actually breathe.

[Michael Stevens]

So the tiny parts of your lungs don't just get stuck together by the water's surface tension. The water is wetter, it's thinner, it flows even more easily.

[Hannah Fry]

Slipperier. Slipperier. Less sticky.

[Michael Stevens]

Slipperier.

[Hannah Fry]

I've loved that so much. That is such a good question. I had no idea. Do they put it in, like, when they're sort of there with the hose, are they pumping out wetter water?

[Michael Stevens]

Sometimes, not. Not all the time. It depends on the type of fire, the type of fuel being burned, but it's a real thing. And so. Jessx, I think I wouldn't have learned any of this it wasn't for your question. I feel like, honestly, I haven't really answered your question because your boyfriend is talking about tap water. And I don't think any municipalities add surfactants and glycols to their drinking water in the tap, but that could go up and down day to day. Or even what's already on your boyfriend's skin might be mixing with that water and creating a more or less wet feel day to day. So that's my guess.

[Hannah Fry]

But wouldn't it be the other way round, though? Because I think, like, feeling wet is about water sticking to you. No. Whereas we're talking about water slipping off.

[Michael Stevens]

Well, I don't know. I would say that actually you're right. Things get wet when water sticks to them. So a hydrophobic substance, like, you know, glass on cars, coated with special things, so the water droplets just come right off. They don't get wet by the water. Where, you know, wet is this verb where wetting means to stick to some metals, like gallium can wet glass, meaning they stick to it. And so the feeling of water being wet. Does wetter water feel less wet?

[Hannah Fry]

I think it would. I think we need to buy some and we need to try this out

[Michael Stevens]

and we need to feel. And then you need to drink it and tell me what it's like. No, but this is sort of two different definitions of wet being used here. One is how quickly does it penetrate materials and the other is how much does it do the opposite, stick to them. And you're right. Wedding is more about Sticking to things and staying on them through service tension.

[Hannah Fry]

We need more information from Jesse. Sex. We need more. I think we need. What does he actually mean?

[Michael Stevens]

Yeah, does he mean that it feels thicker? Cause, you know, what does your boyfriend do? He might be coming into contact with hydrophobic substances like fine dusts that then cause the water to just slough right off his skin and not wet his skin as much. That could be going on.

[Hannah Fry]

Look, I feel like there's, there's a follow up episode on this coming there. You.

[Michael Stevens]

Yeah, I think there is. Okay, so, yeah, keep us, Keep us up to date and we will be following along on the subreddit. Here's a question that came from T senrey. How does space agencies such as NASA communicate with satellites, space probes and rovers in a way where they cannot be hacked? And has this changed over time such that older space probes may be more vulnerable to hacking? Are there any known instances of people hacking satellites?

[Hannah Fry]

Okay, so I had a look into this, right. And I think you would imagine that the older ones would be easier to hack. So, like Voyager 2, for example, because that was launched in 1977, which is, you know, before anyone was worried about cybersecurity, before all of this, like, modern encryption stuff was put out. And I, you know, you probably could. The only slight problem is that it's now 20 billion kilometers away.

[Michael Stevens]

Yeah. You're gonna need a really powerful antenna to, like, contact. I never thought about that, though. Like, clearly NASA communicates it with probably radio waves. Couldn't I spoof that and tell Voyager to, like, come back or something?

[Hannah Fry]

I mean, yes, but you're gonna. I don't know if you could tell it to come back. I'm not sure it's got enough fuel on board. But you could. You, you, you would need to get into NASA's giant deep space network antennas that they have dotted around the world.

[Michael Stevens]

That's the problem. Yeah.

[Hannah Fry]

I mean, they are capable of transmitting these incredibly powerful beams, radio waves, exactly as you described with extraordinary precision. But you're. I mean, no, you're just not going to. You're not going to have one of them sitting around in your backyard. You're not going to be able to build one of them without anyone noticing. And even if you did, you would also need to know Voyager's original language, his programming language, which would be. I mean, who even knows what it was? But it'd be decades old. It would be really. I'm sure there would be details on it out there, but. Don't bother. Basically, what I'M saying is the startup costs are prohibitive and the output that you would get would be also quite limited. Ironically, though, the modern satellites, which are much closer to Earth, sort of makes them slightly easier targets. The thing is, these do have much more standard technology, this sort of encryption that people have authentication because they know that they are easy to intercept. In fact, you know, I can't remember if I've told this story on this podcast before, but, you know, Sputnik, when Sputnik originally was launched, it was a group of students in a bedroom who worked out where Sputnik was because Sputnik was giving out this beep. Have I told you this story before?

[Michael Stevens]

Yeah, yeah, yeah, yeah. But I guess Sputnik wouldn't have the ability to do anything you couldn't, like, tell it to, you know, fall or change its orbit. It just kind of was there and was gonna not be there eventually.

[Hannah Fry]

Exactly right. But it really, I think, demonstrates how these young students were able to tune into it using their radios and to hear the beep that it was giving out, these things are overhead the entire time. It's really not that hard. It really wouldn't be impossible to create an antenna that could tune into what they are putting out and have the original coded message. The problem is, unencrypting it is going to be way, way harder because they're using modern techniques. There have been a couple of mishaps, though. Oh, there was a really Famous one in 2022, 24th of February. This, by the way, is about an hour before Russian tanks crossed the border into Ukraine. So kind of a really critical moment.

[Sponsor/Ad Voice]

Oh.

[Hannah Fry]

But attackers managed to reach the management network of ViaSat's Kasat system. Right. Which is basically. It was a misconfigured vpn. And what they did is they pushed a virus, effectively a wiper called acid rain, out to tens of thousands of modern consumer broadband modems and just bricked them.

[Michael Stevens]

It rained down. So it was like just a vector for this virus.

[Hannah Fry]

Exactly right. Exactly right. You send it up, it sends it down, all of this thing goes. And so what it did was it took out the Ukrainian military satellite communications right at the beginning of the war.

[Michael Stevens]

Wow.

[Hannah Fry]

It also instantly knocked out 5,800 wind turbines in Germany.

[Michael Stevens]

I didn't even know about this.

[Hannah Fry]

Vizat, who are the provider, they had to ship out 30,000 new modems. There was no recovering these things. And also the satellite itself was completely fine. You know, the attack sort of. It just was a little IT system that kind of went up and then and then disseminated to all of the terminals. But, I mean, no one knows who managed to do that. No one knows who carried out that attack. I don't have any suspects. Don't know about you. Not sure.

[Michael Stevens]

No, no, no one's told me.

[Hannah Fry]

Complete mystery. Complete coincidence and complete mystery. But, yeah, it does happen. It's just hard to do. And you have to be, I think, quite motivated. Tell you what, though, quantum computing's coming and encryption's looking a bit ropier.

[Michael Stevens]

Yeah, man, I never even thought about hacking a satellite. Like, could I get. Is Hubble still up there? Could I get Hubble to, like, point down at my house, send you pictures?

[Hannah Fry]

Yeah.

[Michael Stevens]

Take some family photos. I don't think Hubble does well with short distances, but you know what I mean? There are other cameras up there that are pointing at Earth. I could commandeer them and set up some fun photo shoots. With quantum computing, that could become a much bigger problem. I guess you have to hope that encryption keeps up with the computing.

[Hannah Fry]

Yeah, there are ways to do it. There are ways to. I'll be honest with you, Michael. You're starting to sound suspiciously like Mark Zuckerberg wanting to point a mirror to

[Michael Stevens]

his house to give himself day during the night. Yeah, look, I was kidding. I don't really want to do a photo shoot from space.

[Hannah Fry]

That was enjoyable. I really enjoyed those questions. That was a lot of fun.

[Michael Stevens]

They were really good. You guys bring us not just fun questions to answer, but the learning behind how we kind of approach them is just so fun. So thank you and please send in more. We've got our subreddit. The rest is science and the rest is scienceolehanger.com.

[Hannah Fry]

do you worry, though, that we research for these every single week?

[Sponsor/Ad Voice]

Right.

[Hannah Fry]

Do you worry that your brain is going to get to a point where it's like, no, I'm actually. I'm actually full now.

[Michael Stevens]

No, I don't think so, but I think that it might get so full that, like, imagine that no one can stump us. They're just like, hey, what if human hair was made of spaghetti? And we're like, oh, my gosh, this one again? Yeah, look, you know, every week you'd get 2,000 calories of hair and blah, blah, blah, blah, blah, blah, blah, and we would just become know it alls to a, like, boring extent.

[Hannah Fry]

I strongly suspect we might already be there, Michael, but. But I think there's still. There's still mileage in this. Thank you so much for joining us. We will be back with another few episodes next week. Bye. Bye.

[Michael Stevens]

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