The Rest Is Science
Episode: "The Smell of Christmas Is Tree Screams" (December 25, 2025)
Hosts: Professor Hannah Fry & Michael Stevens (Vsauce)

Episode Overview

In this festive episode of The Rest Is Science's "Field Notes," Professor Hannah Fry and Michael Stevens (filling in for Michael Blastland) dive into the surprisingly twisted science behind some beloved holiday traditions, tactile bodily illusions, and the quirky potential of elemental baby names. Recorded as a special for Christmas Day, the conversation gleefully balances playful banter with deep dives into evolutionary biology, neuroscience, and social perception. The main throughline centers on what Christmas trees are actually "saying" when they perfume our homes, using their scent as a jumping-off point to explore evolutionary strategies, human senses, and the difference between mental maps and reality.

Key Discussion Points & Insights

1. Why Christmas Trees Smell the Way They Do

(Starts ~04:07)

• Rowan, a listener, asks: "Why do Christmas trees smell the way they do? Is there an evolutionary reason for producing those chemicals?"
• Hannah Fry explains:
  • The iconic scent from Christmas trees comes from a group of molecules called terpenes, specifically pinene.
  • The smell is not intended to make humans feel festive:

    "Basically the smell of fear, Michael, that's essentially what we are, what we’re smelling." (06:21)

  • For trees, this smell acts as a defense mechanism:
    • It is toxic or deeply unpleasant to insects and fungi—for pine beetles and other pests, it’s “chemical warfare.”
    • The scent is a damage response: when branches are snapped or needles crushed, more pinene is released.
    • There is evidence that trees may use these chemicals to communicate warning signals to other trees.
  • Ultimately:

    "What you’re doing then is you’re...sort of smelling the tree screaming, you know?” (07:01)

• Michael Stevens (Vsauce) responds:

  "It's a wonderful, festive smell, but it's the scream of a tree." (08:00)

Festive Evolution & Human Resonance

• Fry elaborates that humans may be evolutionarily primed to find pine scent appealing, as ancient humans would associate it with shelter, safety, and lack of insects.
• Michael:

  "It really is literally the smell of safety." (11:25)

• Hannah adds:

  "Yeah, and screams!" (11:29)

Flammability: An Evolutionary Fire Strategy

• Fry reveals that the same chemicals making pine “scented” also make them highly flammable:

  "It feels really counterintuitive...if you have a low-level fire in a forest, one that is very quick and runs through, then it will just clear out, like, competition...So in evolutionary terms, being a little bit explodey is actually quite a good long-term strategy." (09:54)

• Michael sums up:

  "That's a good life lesson." (10:34)

2. Elemental Baby Names

(Starts ~11:34)

• Inspired by a listener's encounter with someone named Xenon, Fry and Michael riff on which elements from the periodic table would make the best human names.
• Michael’s take:
  • Names like Xenon, Neon, Tungsten, and Indium are fun, but remarks some might be "a bit much."
  • Suggests an element named after John Quincy Adams to finally integrate "J" and "Q" into the periodic table:

    "If we...could fix that in one fell swoop by naming an element after John Quincy Adams...Let’s get some J's and Q's in there and then the whole alphabet will be represented." (13:20)

• Hannah’s playful British alternative:
  • Using regional accents, elements like “Helium” become “Hey, Liam,” and “Argon” as “Are Gone.”

    "Periodic table as British regional accent pet names. I think we're already there." (14:21)

3. Tactile Illusions & The Brain’s Map of the Body

(Starts ~18:36)

The Upside-down Tongue Illusion

• Michael leads listeners through a "hands-on" experiment:
  1. Twist your tongue upside down (manually or with fingers).
  2. Touch the tip from left to right.
  3. You can't accurately sense which side is being touched.

  "There's actually an illusion happening here which is that you can't locate which side of your tongue is being touched." (20:08)

  • Brain is unaccustomed to this configuration and projects touch sensations inaccurately.

Aristotle’s Finger Illusion

• Cross your pointer and middle finger and touch an object between the tips.
  • Feels as if two objects are present due to brain misinterpreting the now-adjacent "outer" edges.

    "It's called Aristotle's Illusion. And it's very fun to cross your fingers and just touch things...It feels like two very separate things when it’s really just one." (22:48)

The Warped Map of Sensation

• Hannah shares a British playground trick: tracing a finger up someone’s arm with their eyes closed, they’ll think you reach their elbow much sooner because the brain “stretches” sensation at different rates across the body.

  "Your mental map is warped. It's a bit like a London tube map in a way. You know, very good for navigation but terrible for accuracy." (25:54)

• Michael likens these moments to the jolt of an unmoving escalator: the brain’s “prediction” clashes with experience.

4. The Bayesian Brain & Perceptual Plasticity

(~26:34 onward)

• Fry and Michael discuss the “Bayesian brain”—the idea we are always predicting and modeling the world based on incoming sensory and prior information, never accessing “reality” directly.

  "We're not actually a body. We're sort of...processing engine that's locked inside a dark, noiseless void inside our skulls." (26:43)

• Michael:

  "At night...close your eyes in the dark and think about how much darker it is in your head. Everything you're hearing is all just your brain putting that show on for you." (27:19)

Phantom Limbs, Remapping, and Technological “Vision”

• Hannah describes her cousin’s and Admiral Nelson’s experiences with phantom limbs—they felt sensations in limbs no longer present.
  • Nelson considered this evidence of the soul.
• Fry recounts an experiment where the brains of newly blind individuals "remap" visual input delivered via tactile signals on the tongue. Over time, they report “seeing” through their tongues.

  "You could, for example, throw a ball at somebody who no longer had vision but was wearing this tongue plate. And they could...see the ball coming towards them." (34:19)

• Takeaway: The brain can adapt its maps, given new input.

5. Memorable Quotes & Moments

• Hannah Fry: "Basically the smell of fear, Michael, that's essentially what we are, what we're smelling." (06:21)
• Michael Stevens: "It's a wonderful, festive smell, but it's the scream of a tree." (08:00)
• Hannah Fry: "Periodic table as British regional accent pet names. I think we're already there." (14:21)
• Michael Stevens: "Our mental map in our head is the correct one. The problem is everything else." (31:06)
• Hannah Fry: "Reality is just the hallucination we all agree on." (32:04)

Important Timestamps

• 04:07 — Introduction of the Christmas tree smell question
• 06:21 — The smell of Christmas trees as “tree screams"
• 09:54 — Evolutionary strategy: explosive, flammable trees
• 11:34 — Elemental baby names discussion
• 18:36 — Tactile illusions: The upside-down tongue
• 22:21 — Aristotle’s Illusion (crossed fingers)
• 25:54 — How the brain warps the body map
• 26:34 — Discussion on “the Bayesian brain”
• 34:19 — Remapping vision via the tongue in blind individuals
• 35:02 — The adaptability and plasticity of the brain

Tone & Style

• Playful and witty, with a penchant for deadpan British humor and Vsauce-style curiosity.
• Rich metaphors (e.g., London tube map for the body’s perception).
• Inclusive: Both hosts encourage listeners to participate in the tactile experiments and invite audience questions for future episodes.
• Seriously silly: Deep dives into neuroscience follow jokes about “tree screams” and “explosive personalities.”

Conclusion

This episode connects nostalgic Christmas scents to the realities of evolutionary warfare, uses bodily illusions to unpick the relationship between our brains and the world, and rounds things off with joyful speculation about baby names from the periodic table. The big message: reality is more fragile, weird, and wonderful than we think—and it all starts with questioning the things we take for granted, even on Christmas Day.