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.
Showing 67 oddities.
This tag marks entries the reader can safely test on themselves right now. Tongue rolling, the blind spot, and afterimages all come with a simple way to notice the effect directly.
The tag exists because some things in this catalogue are far more convincing in person than in a description. A reader here usually wants a quick, safe demonstration, not just an explanation.
This list will tell you what to do and what you should notice if it works. It will not guarantee the effect for every reader, since some tricks and perceptions only work for part of the population. If nothing happens, that is itself part of what the entry is describing.
Spiritual 5Icky 5Not an emergency 21Get it checked 15Try it 67Learnable 18Party trick 19Vestigial 7Genetic 37Sleep 6Illusion 22Childhood 8Debunked myth 11Community 14Famous people 14Record 7
Results narrow the list below as you type.
Brain Trick
Ambidexterity
Using both hands equally well for every task. About 1% of people are truly ambidextrous. Many people who say they are ambidextrous are mixed-handed: right hand for some jobs, left for others.
About 1%
Brain Perception
Aphantasia
Most people see a picture in their mind when they think of a friend's face. People with aphantasia see only a faint image or none. About 4 in 100 people are like this, and about 1 in 100 see nothing at all. A few people have images almost as vivid as real sight.
About 1% have no mental images. About 4% have faint or none.
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.
About 61% attached in one Korean sample
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. The arcs follow the paths of the nerve fibres in your own retina.
94% of healthy eyes in one clinic test
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.
Most people
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 usually passes in under a minute.
About 40–50% of people
Bones Variation
Camptodactyly
A finger that stays bent forward at its middle joint and cannot fully straighten, usually the little finger. Under 1% of people have it. Mild cases rarely cause trouble, but the bend can get worse while a child grows.
Under 1% of people
Brain Perception
Chronostasis
Glance at a clock and the second hand can seem 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. Almost everyone sees the speckle, and many people see more. Almost nobody talks about them.
Almost everyone
Mouth Trick
Cloverleaf tongue
Folding the tongue into three lobes, like a clover leaf. In one Dutch study, about 15% of adults could do it. It is usually called the hardest tongue shape.
About 15% (one Dutch sample)
Brain Reflex
Contagious yawning
Seeing, hearing, or even reading about a yawn makes many adults yawn. The effect appears in early childhood, may be linked to empathy, and crosses over to dogs.
About 30–70% of adults
Eyes Trick
Crossing your eyes
Turning both eyes inward toward the nose on purpose. Most people with normal two-eyed vision can do it. Your eyes will not get stuck that way.
Most people (no formal count)
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 first pregnancy and in some men. A bulge down the midline when you sit up is the usual sign.
Almost all first 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 studied so far has it. You can trigger it with a bowl of cold water.
Everyone
Ears Sound
Ear clicking when swallowing
Swallowing 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
Up to 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. About 1 in 5 people can wiggle their ears. Some people can learn.
About 1 in 5
Eyes Perception
Eigengrau
The colour you see in total darkness is not black. It is a very dark grey, most likely 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. Nobody has counted how many people can do 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. No study has counted how many people can do it on command. It also happens on its own during hard breathing and in strong emotion such as anger.
No formal count
Brain Perception
Ganzfeld effect
Fill your whole field of view with a featureless colour and, after some minutes, your brain may start 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, in lab tests
Mouth Trick
Gleeking
Shooting a thin jet of saliva from under the tongue. It can happen by accident, often during a yawn, or on purpose. No survey has counted how many people can do it.
No formal count 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 Icky
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. Breathing can shift it a little within minutes. A few trained people shift it a lot.
No formal count
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 in the US
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 ten weeks of training.
Nobody has counted it
Eyes Trick
Independent eye movement
Moving one eye while the other stays still, or moving the two eyes in different directions. Few people seem to manage it, and nobody has counted them. Chameleons do it all day.
No formal count
Brain Involuntary
Jamais vu
A familiar word, face or place suddenly feels strange, as if you are meeting it for the first time. It is the opposite of déjà vu, and researchers think it is rarer. In lab studies, more than half of people felt a common word turn strange after copying it about 30 times.
Unknown. Thought to be rarer than déjà vu.
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 a classic example of a one-synapse reflex.
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 pop comes from a gas bubble in the joint fluid, and it does not cause arthritis.
25–54% crack habitually
Muscle Variation
Linburg–Comstock variation
An extra tendon link in the forearm that ties the thumb to the index finger. When the thumb bends into the palm, the index fingertip bends with it. Roughly 1 in 5 people have it, but counts vary widely.
About 1 in 5 people. Studies range from 5% to 60%.
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 12 people have it. It looks like the midfoot bend of apes.
About 1 in 12 (8%)
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 about 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 15–20% 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. Pressure on the eye makes the retina fire without any light.
Anyone with a working retina
Nerves Involuntary
Pins and needles
The prickling, buzzing feeling in a limb after you sit on it or lean on it. It happens when blood returns to a squeezed nerve and its sensory fibres briefly fire on their own.
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. Nobody has counted how many can flex it on its own.
No formal count
Mouth Trick
Pointed tongue
Making the tip of the tongue narrow and sharp, like a pencil point. About 55% of people in one Chinese survey can do it. It and tongue rolling are the two most common tongue tricks.
~55%
Brain Perception
Prosopagnosia
People with prosopagnosia, or face blindness, find it hard to recognise faces, even those of family and friends. Most have had it all their lives. It can also start after brain damage. Estimates run from about 1 in 100 people to about 1 in 40, depending on how it is tested.
About 1–2.5% of people
Skin Involuntary
Pruney fingers
Fingertips and toes wrinkle after several minutes in water. This is not just the skin soaking up water. It is the nervous system shrinking the fingertip on purpose, and the wrinkles may improve grip when wet.
Nearly 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 pupils react together. 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. Informal estimates say 30% to 35% of people can do it without training. No study has measured how many of the rest can learn it.
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.
60–80% 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 Variation
Supertasters
About a quarter of people taste bitter compounds far more strongly than everyone else. They tend to have more taste buds, 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. Studies find that between 1.5% and 27.5% of people can do it.
1.5–27.5%
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. Nobody has counted how many people can do it. The loudest recorded burp reached 112.4 decibels.
No formal count
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. Some divers and freedivers learn it, but most never do.
No formal count
Eyes Trick
Voluntary nystagmus
About 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.
About 8% of people
Skin Trick
Voluntary piloerection
Giving yourself goosebumps on command, without music, cold, or fear. Nobody knows how many people can do it, but it seems rare. The muscles involved are not supposed to take orders.
Unknown, but rare
Eyes Trick
Voluntary pupil control
Changing the size of your pupils by will alone, without imagining a light or doing sums. One case has been studied with modern methods. Older reports exist, but most describe indirect tricks. Indirect control by thinking hard is possible for almost everyone.
One modern case study. No rate known
Eyes Trick
Wall-eyed divergence
Turning both eyes outward, away from the nose, on purpose. Nobody has counted how many people can do it. Most people can learn the mild version, parallel viewing, from Magic Eye pictures.
Not known