Eyes Perception
Afterimages
An image that stays in your vision after the thing you looked at is gone. Bright things leave a same-colour copy for a moment. Long stares leave a copy in opposite colours.
Everyone
Try it
You can test this on yourself right now.
62 oddities.
Spiritual 6Icky 4Not an emergency 20Get it checked 14Try it 62Learnable 20Party trick 19Vestigial 7Genetic 35Sleep 6Illusion 21Childhood 10Debunked myth 11Community 13Famous people 14Record 7
Brain Trick
Ambidexterity
Using both hands equally well for every task. Fewer than 1% of people are truly ambidextrous. Most people who say they are ambidextrous are mixed-handed: right hand for some jobs, left for others.
Under 1%
Ears Variation
Attached and free earlobes
Some earlobes hang free and others join the head directly. Textbooks taught this as one gene, but a 2017 study found at least 49 genetic regions involved, and the two shapes blend into each other.
Both shapes are common
Mouth Variation
Bifid uvula
The uvula, the dangling flap at the back of the throat, is split into two lobes, in about 1 to 2% of people. On its own it is harmless, but in a baby it can be a clue to a hidden cleft of the palate.
1–2% of people
Eyes Perception
Blue arcs of the retina
Look at a small red light in a dark room and two faint blue arcs curve away from it towards your blind spot. You are seeing the nerve fibres of your own retina light up.
Most people, in the right conditions
Eyes Perception
Blue field entoptic phenomenon
Tiny bright dots that dart along wiggly paths when you look at a clear blue sky. They are your own white blood cells moving through the capillaries of the retina.
Everyone
Nerves Involuntary
Brain freeze
A short, sharp headache in the forehead a few seconds after cold food or drink touches the roof of the mouth. It passes in under a minute.
About 30–40% of people
Brain Perception
Chronostasis
Glance at a clock and the second hand seems to freeze for a moment before it moves. The brain hides your eye movement and stretches the first thing you see to cover the gap.
Everyone
Bones Variation
Clinodactyly
A finger that curves sideways toward its neighbours, most often the little finger leaning toward the ring finger. Between 1% and 20% of people have it, and on its own it is harmless.
1–20% of people
Eyes Perception
Closed-eye hallucinations
Close your eyes and you do not see nothing. You see speckle, drifting colour, and, as you fall asleep, moving patterns and even scenes. Everyone has them. Almost nobody talks about them.
Everyone
Mouth Trick
Cloverleaf tongue
Folding the tongue into three lobes, like a clover leaf. Between 10% and 15% of people in Western samples can do it. It is usually called the hardest tongue shape.
10–15% (Western samples)
Brain Reflex
Contagious yawning
Seeing, hearing, or even reading about a yawn makes about half of adults yawn. The effect appears in early childhood, is linked to empathy, and crosses over to dogs.
About 50–60% of adults
Eyes Trick
Crossing your eyes
Turning both eyes inward toward the nose on purpose. Nearly everyone with normal two-eyed vision can do it. Your eyes will not get stuck that way.
Nearly everyone
Ears Variation
Darwin's tubercle
A small bump on the folded rim of the ear, which Darwin thought was the leftover tip of a pointed ancestral ear. Between 10% and 58% of people have one, depending on the population.
10–58% of people
Skin Reflex
Dermatographia
In a few people, a firm stroke on the skin raises a red welt within minutes, clear enough to write words with a fingernail. It is the skin's hive response set on a hair trigger.
About 2–5% of people
Muscle Variation
Diastasis recti
A widening of the gap between the two halves of the six-pack muscle, seen in nearly every pregnancy and in some men. A bulge down the midline when you sit up is the usual sign.
Almost all pregnancies; about 1 in 3 women a year after birth
Heart Reflex
Diving reflex
Cold water on the face makes the heart slow down, sends blood to the brain, and holds the breath. Every mammal has it. You can trigger it with a bowl of cold water.
Everyone
Ears Sound
Ear clicking when swallowing
Every swallow opens a small tube between your throat and your middle ear for a fraction of a second. The faint click you can hear is that tube letting air through to balance the pressure.
Everyone
Ears Trick
Ear rumbling
About half of all people can make a low roar inside their own head by tensing a tiny muscle in the middle ear.
43–55% of people
Ears Trick
Ear wiggling
Moving the outer ears using three small muscles that most mammals use to point their ears at sounds. Between 10% and 22% of people can wiggle at least one ear. Many more can learn.
10–22% (one ear)
Eyes Perception
Eigengrau
The colour you see in total darkness is not black. It is a very dark grey, because the light-sensing cells in your eyes fire a little even when no light reaches them.
Everyone
Eyes Trick
Eyelid flipping
Turning the upper eyelid inside out so the pink inner surface shows. Most people with reasonably loose lids can learn it. Eye doctors do it to patients to look for grit.
No formal count Icky
Muscle Trick
Flaring your nostrils
Widening the nostrils on purpose. About 30% of people can do it on command. Everyone does it without thinking during hard breathing and strong emotion.
~30% on command
Brain Perception
Ganzfeld effect
Fill your whole field of view with a featureless colour and, within minutes, your brain starts inventing shapes, lights, and scenes. Two halved ping-pong balls and a lamp are enough to do it at home.
Most people who try it
Mouth Trick
Gleeking
Shooting a thin jet of saliva from under the tongue, sometimes a metre or more. About 35% of people do it by accident now and then. Only about 1% can do it reliably on command.
~1% on command Icky
Mouth Trick
Gorlin's sign
Touching the tip of your nose with your tongue. About 10% of people can do it. About half of people with Ehlers-Danlos syndrome can, so doctors use it as a quick check for stretchy connective tissue.
~10% of people
Eyes Perception
Haidinger's brush
Humans can see the polarization of light without any tool. It appears as a faint yellow bow-tie in the middle of your vision when you look at a white screen or a blue sky.
Most people, with practice
Heart Trick
Heart rate control
Slowing or speeding up your own heartbeat on purpose, using breath, attention, or training. Most people can shift it a little within minutes. A few trained people shift it a lot.
Most people, a little
Bones Variation
Hitchhiker's thumb
A thumb whose top joint bends back past 50 degrees, once taught as a simple recessive gene. Thumb angle is really a smooth range, and the genetics are not that simple.
24–36% of people
Brain Rare ability
Human echolocation
Some blind people find their way by making sharp tongue clicks and listening to the echoes. Brain scans show the echoes are processed in the visual cortex. Sighted people can learn a basic version in a few weeks.
Rare, mostly among blind people
Eyes Trick
Independent eye movement
Moving one eye while the other stays still, or moving the two eyes in different directions. Very rare in humans. Chameleons do it all day.
Very rare; no formal count
Bones Variation
Joint hypermobility
About 1 in 5 people have joints that bend further than usual, measured with a nine-point test called the Beighton score. For most people it causes no trouble.
10–25% of people
Nerves Reflex
Knee-jerk reflex
A tap on the tendon below the kneecap makes the lower leg kick. The signal goes to the spinal cord and straight back through one connection. It is the simplest reflex in the body.
Everyone
Bones Trick
Knuckle cracking
Pulling or bending a finger joint until it pops. Between 25% and 54% of people do it out of habit. The sound is a gas bubble forming in the joint fluid, and it does not cause arthritis.
25–54% crack habitually
Bones Trick
Midtarsal break
A foot that bends in the middle, between the heel and the ball, as well as at the toes. About 1 in 13 people have it. It is a leftover from tree-climbing ancestors.
About 1 in 13 (7.7%)
Bones Variation
Morton's toe
A second toe that looks longer than the big toe because the first metatarsal bone in the foot is short. It is common, and it is the foot shape seen on classical Greek statues.
3–22% of people
Brain Perception
Motion aftereffect
Watch something move in one direction for half a minute, then look at something still. The still thing appears to drift the opposite way for a few seconds.
Everyone
Muscle Variation
Palmaris longus
A thin tendon that stands out in the wrist when you press your thumb and little finger together. About 1 in 7 people do not have it at all, and they lose nothing.
Missing in about 15% of people
Eyes Involuntary
Phosphenes
The flashes, rings, and patterns of light you see when you rub or press your closed eyes, or after a knock on the head. The retina fires from pressure instead of light.
Everyone
Nerves Involuntary
Pins and needles
The prickling, buzzing feeling in a limb after you sit on it or lean on it. It happens when pressure comes off a nerve and the nerve fibres restart at different speeds.
Everyone
Muscle Trick
Platysma display
Tensing the thin sheet of muscle under the skin of the neck so that cords stand out from jaw to collarbone. Everyone has the muscle. Isolated control of it is uncommon.
No formal count
Mouth Trick
Pointed tongue
Making the tip of the tongue narrow and sharp, like a pencil point. About 55% of people can do it. After rolling, it is the most common tongue trick.
~55%
Skin Involuntary
Pruney fingers
Fingertips and toes wrinkle after 5 to 15 minutes in water. This is not the skin soaking up water. It is the nervous system shrinking the fingertip on purpose, and the wrinkles may improve grip when wet.
Everyone with intact nerves
Eyes Reflex
Pupillary light reflex
Shine a light into one eye and both pupils shrink, even the eye in the dark. The wiring crosses in the midbrain so that the two eyes always match. You can watch it in a mirror.
Everyone
Eyes Perception
Purkinje effect
As the light fades at dusk, red things go dark first and blue things stay bright. Your eyes switch from colour-sensing cones to blue-favouring rods, and the whole world shifts towards blue.
Everyone
Eyes Perception
Purkinje tree
A branching shadow of your own retinal blood vessels. It is always on your retina, but you only see it when light enters the eye from an unusual angle and the shadow moves.
Everyone
Muscle Trick
Raising one eyebrow
Lifting a single eyebrow while the other stays down. Between 30% and 35% of people can do it without training. Most of the rest can learn it in front of a mirror.
30–35%
Brain Involuntary
Rubber hand illusion
Stroke a rubber hand and a person's hidden real hand at the same time, and within a minute or two the brain adopts the rubber hand as its own. It shows that your sense of your body is built moment by moment from sight and touch.
70–80% of participants
Brain Perception
Strange-face illusion
Stare at your own face in a mirror in dim light for a few minutes and it starts to distort. Most people see their features warp. Some see a stranger, an animal, or a monster.
66% see their face distort
Mouth Rare ability
Supertasters
About a quarter of people taste bitter compounds far more strongly than everyone else. They have more taste buds and a particular version of one gene, and they often dislike coffee, dark greens, and grapefruit.
About 25% of people
Eyes Perception
The blind spot
Each eye has a hole in its field of view where the optic nerve leaves the retina. You never notice it, because the brain paints over the gap with whatever surrounds it.
Everyone
Mouth Trick
Tongue folding
Folding the tip of the tongue backward over itself inside the mouth. Between 14% and 27.5% of people can do it.
14–27.5%
Mouth Trick
Tongue rolling
Between 65% and 84% of people can curl the sides of the tongue up into a tube. For decades it was taught as a simple one-gene trait. It is not.
65–84% of people
Mouth Trick
Tongue twisting
Turning the tongue on its side so the edges face up and down, or all the way over. Between 12% and 36% of people can twist it at least halfway.
12–36%
Bones Variation
Torus palatinus
A hard bony lump along the midline of the roof of the mouth that grows slowly and is completely harmless. Many people find it by chance and worry that it is a tumour.
About 20% of people
Eyes Perception
Troxler effect
When you hold your gaze on one point, things in the edge of your vision fade out and disappear. The brain stops reporting anything that does not change.
Everyone
Organs Trick
Voluntary burping
Swallowing air on purpose and letting it back up as a belch. Most people can learn it. The loudest recorded burp is louder than a motorcycle.
Most people can learn it
Ears Trick
Voluntary Eustachian tube opening
Opening the tubes that link the middle ear to the throat on command, without swallowing or pinching your nose. You hear a click, and your own voice gets loud. Divers and frequent flyers learn it.
Many can learn it; no formal count
Eyes Trick
Voluntary nystagmus
Between 5% and 8% of people can make both eyes shake rapidly from side to side on command. It looks alarming and is harmless. It is tiring and lasts only seconds.
5–8% of people
Skin Trick
Voluntary piloerection
Giving yourself goosebumps on command, without music, cold, or fear. About 1 in 1,500 people can do it. The muscles involved are not supposed to take orders.
About 1 in 1,500
Eyes Trick
Voluntary pupil control
Changing the size of your pupils by will alone, without imagining a light or doing sums. Only a handful of cases are documented. Indirect control by thinking hard is possible for almost everyone.
A handful of documented cases
Eyes Trick
Wall-eyed divergence
Turning both eyes outward, away from the nose, on purpose. It is much rarer than crossing the eyes. Most people can learn a mild version through Magic Eye pictures.
Rarer than crossing; no formal count