Bones Variation
Accessory ossicles
Small extra bones, most often in the foot, that form when a growth centre never fuses with its main bone. They are harmless but can be mistaken for fractures on an X-ray.
Up to 1 in 5 feet
Bodily Oddities
Strange things bodies (can) do.
140 oddities catalogued so far.
Results appear below as you type.
Browse by body region
All oddities
140 Eyes Reflex
Accommodation reflex
When you look at something close, three things happen at once. The eyes turn inward, the pupils shrink, and the lens in each eye gets fatter. You do not decide any of it.
Everyone
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
Brain Perception
Alice in Wonderland syndrome
Objects, body parts, or the whole room suddenly look far too big, far too small, or far away. It mostly happens to children, often during a fever or infection, and it passes on its own. In adults it is usually linked to migraine.
6–7% of teenagers (size distortion)
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%
Brain Involuntary
ASMR
A pleasant tingling that starts on the scalp and moves down the neck and spine, set off by whispering, tapping, soft sounds, or gentle personal attention. Not everyone gets it. A large online community is built around it.
Unclear; estimates 20–70%
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
Ears Sound
Autophony
Your own voice, breathing, or heartbeat sounds far too loud inside your head. It usually means a tube or a thin patch of bone is letting sound take a shortcut into the ear.
About 0.3–6.6% for patulous Eustachian tube
Nerves Reflex
Babinski reflex
Stroke the sole of a baby's foot and the big toe lifts while the other toes fan out. Do the same to an adult and the toes curl down. The baby's response is normal until about two years. In an adult it is a warning sign.
All healthy infants
Muscle Involuntary
Benign fasciculations
Small visible twitches in the eyelid, calf, thumb, or arm that come and go with no weakness. About 70% of healthy people get them. They are not a sign of nerve disease.
About 70% of healthy people
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
Skin Reflex
Blushing
When you are embarrassed, blood vessels in the face open wide and the skin turns red. You cannot stop it. Darwin called it the most human of all expressions, and no other animal seems to do it.
Everyone
Ears Involuntary
BPPV
A short, intense spinning feeling set off by turning over in bed or tipping your head back. Tiny crystals in the inner ear have drifted into the wrong place. It is the most common cause of vertigo and it is very treatable.
About 2.4% of people, lifetime
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 Involuntary
Brain zaps
A brief feeling like a small electric shock inside the head, often set off by moving the eyes. It is most common when stopping or lowering an antidepressant. It is unpleasant but not dangerous.
Common when stopping antidepressants
Brain Involuntary
Call of the void
A sudden, brief thought of jumping when you stand somewhere high, or of swerving while driving, with no wish to do it. About half of people get it. It is the brain's safety system being misread, not a hidden wish.
43–60% of people
Body Rare ability
CCR5-delta32
About 1% of people of Northern European descent carry two copies of a broken gene that makes them almost immune to the most common form of HIV. A bone marrow transplant from one such donor produced the first cure of HIV.
About 1% of Northern Europeans (two copies)
Bones Variation
Cervical rib
An extra rib above the normal top rib, growing from the lowest bone in the neck. About 1 in 100 people have one, and most never know.
About 1% of people
Brain Perception
Charles Bonnet syndrome
People who lose a lot of their sight often start to see things that are not there: patterns, faces, small people in costume, whole landscapes. They know the images are not real. It is caused by the eyes, not the mind.
10–40% of people with serious sight loss
Body Rare ability
Chimerism
A chimera is one person made of two sets of cells with different DNA. It happens when two embryos fuse in the first days of pregnancy. Most chimeras never find out. A few discovered it when a DNA test said they were not the mother of their own children.
Unknown. About 100 documented cases
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)
Organs Reflex
Cold diuresis
Being cold makes you need to urinate. The body pulls blood away from the skin into the core, the heart reads this as too much blood, and the kidneys are told to dump water.
Everyone
Nerves Rare ability
Congenital insensitivity to pain
A tiny number of people are born unable to feel pain. Single-gene changes switch off their pain nerves. It is not a superpower. Injuries go unnoticed, and most people with it are badly hurt in childhood.
About 1 in 1,000,000
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
Nerves Reflex
Cutaneous rabbit illusion
A few quick taps at the wrist followed by taps at the elbow feel like a small animal hopping up the arm. The brain fills in taps between the two spots that never happened.
Works on most people tested
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
Brain Involuntary
Déjà vu
A strong feeling that you have lived the present moment before, even though you know you have not. Almost everyone gets it. It peaks between the ages of 15 and 25.
60–97% of people
Brain Involuntary
Derealization and depersonalization
The world suddenly seems flat, distant, or like a film (derealization), or you feel like you are watching yourself from outside (depersonalization). Most people have a brief episode at some point, usually under stress.
50–75% of people, at least once
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
Eyes Variation
Distichiasis
A second row of eyelashes growing from the oil-gland openings along the inner edge of the eyelid. It is rare, mostly inherited, and Elizabeth Taylor was born with it.
Rare
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)
Brain Rare ability
Eidetic memory
Some children can keep a detailed picture of an image in view for a short time after it is taken away. The ability fades by the teenage years. A true photographic memory in adults has never been confirmed.
2–10% of children aged 6–12
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 Reflex
Emotional tears
Humans are the only animals known to weep from feeling. Emotional tears have a different chemistry from the tears that wash the eye, and babies cannot make them until a few months after birth.
Everyone
Brain Involuntary
Exploding head syndrome
A loud bang, crash, or explosion heard inside the head while falling asleep or waking up. Nothing actually happens, there is no pain, and it is harmless.
10–37% of people, lifetime
Organs Rare ability
Extraordinary breath-holding
Most adults can hold their breath for 30 to 90 seconds. Trained freedivers manage over 11 minutes, and over 24 minutes after breathing pure oxygen. One group of sea nomads has evolved larger spleens for diving.
Trained divers only
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
Eyes Involuntary
Floaters
Small grey threads, dots, or cobwebs that drift across your vision and dart away when you look at them. They are shadows of tiny clumps inside the eye. They are common and usually harmless.
About 70% of people, at some point
Skin Involuntary
Fordyce spots
Tiny pale yellow or white bumps, 1 to 3 mm across, on the edge of the lips or inside the cheeks. They are ordinary oil glands in a place without hair. Most adults have them.
70–80% of adults
Eyes Perception
Foveal tritanopia
The very centre of your vision cannot see blue. The middle of the fovea has no blue-sensitive cones at all. You never notice, because the brain fills the colour in from the surrounding area.
Everyone
Brain Involuntary
Frisson
Goosebumps and a wave of chills down the back at a powerful moment in music, a film, or a speech. About half of people get it regularly. It comes from the brain's reward system reacting to an expectation being met or broken.
About 50% of people, regularly
Mouth Reflex
Gag reflex
Touch the back of the tongue or the soft palate and the throat clamps shut. It keeps things out of the airway. Up to a third of healthy people barely have it, and that is normal.
Present in about 2 of 3 people, weak or absent in 10–37%
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 Variation
Geographic tongue
Smooth red patches with pale borders that form a map on the tongue and move around over days and weeks. About 1 to 3% of people have it, and it is harmless.
1–3% of people
Brain Perception
Geometric nightmares
A kind of dream, usually during fever in childhood, filled with huge or tiny shapes, infinite space, crushing pressure, and dread. The name comes from an online forum, not a medical text, but each piece of it is well known to neuroscience.
Unknown. Common in children with fever
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
Skin Reflex
Goosebumps
Tiny muscles at the base of every hair pull the hair upright when you are cold, scared, or moved. In a furry animal this traps warm air or makes it look bigger. In a human it does almost nothing.
Everyone
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
Eyes Variation
Heterochromia
Eyes of two different colours, or one eye with two colours, in fewer than 1 in 100 people. When it is present from birth it is almost always harmless.
Under 1% of people
Organs Sound
Hiccups
A hiccup is a sudden jerk of the diaphragm followed by the voice box snapping shut. Everyone gets them, even before birth. The record case lasted 68 years.
Everyone
Brain Rare ability
Highly superior autobiographical memory
A few dozen people worldwide can recall what happened on almost any date of their adult lives, in detail and without effort. Their memory for other things, such as lists or numbers, is ordinary.
About 62 documented cases
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
Brain Rare ability
Human magnetoreception
Birds, turtles, and many other animals sense Earth's magnetic field. Whether humans do is unsettled. A 2019 experiment found brain-wave changes in response to a rotating field, but nobody has shown a conscious magnetic sense.
Unknown
Brain Involuntary
Hypnagogic hallucinations
Vivid images, sounds, or touches that appear while you are falling asleep or waking up. A face, a voice saying your name, a hand on your shoulder. They are dream fragments arriving early or late, and they are normal.
About 25–37% of people
Brain Involuntary
Hypnic jerks
A sudden jolt of the arms, legs, or whole body just as you fall asleep, often with a feeling of falling. Most people get them, and they are harmless.
60–70% of people
Mouth Involuntary
Inability to burp
Some people have never burped in their lives. Gas cannot pass a ring of muscle at the top of the food pipe, so it stays trapped. This causes gurgling, bloating, and painful pressure. Since 2019 it has a name and a treatment.
Unknown; probably uncommon
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
Mouth Trick
Khechari mudra
A hatha yoga practice in which the tongue is turned back and up behind the soft palate, into the space at the back of the nose. Very few people can do it without months or years of stretching.
Very rare without training
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
Brain Perception
Meditation body dissolution
During long, still meditation the brain loses track of the arms and the edges of the body. Hands can feel close to the shoulders and far away at the same time. Flotation tanks produce the same effect. It is common, harmless, and well studied.
Common in long-term meditators
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%)
Brain Involuntary
Migraine aura
A shimmering zigzag or C-shaped patch of light that grows across your vision over 5 to 60 minutes and then fades. It can come before a migraine headache or, in some people, with no headache at all.
About 3% of people
Brain Rare ability
Mirror-touch synesthesia
About 1 in 60 people feel a touch on their own body when they watch someone else being touched. The brain system that mirrors other people's actions appears to be turned up too high.
About 1.6% of people
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
Brain Sound
Musical ear syndrome
Some people with hearing loss hear clear, complete music that no one is playing. The brain replays stored songs to fill the silence. The person knows the music is not real.
About 2–3% of people with profound hearing loss
Eyes Perception
Palinopsia
An image that keeps appearing after the thing you saw is gone. Brief trails behind moving objects are common and harmless. Copies that persist or return minutes later need a doctor.
Brief trailing is common. Persistent forms are rare.
Nerves Reflex
Palmar grasp reflex
Put a finger in a newborn's palm and the fingers close around it, hard. The grip is strong enough to hold the baby's weight for a moment. It fades by about six months and is replaced by grasping on purpose.
All healthy newborns
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
Nerves Involuntary
Pee shivers
A brief involuntary shudder during or just after urinating. Most men and many women get it. It is widely known and almost completely unstudied.
About 83% of men, 58% of women
Brain Rare ability
Perfect pitch
About 1 in 10,000 people can name or sing any musical note without hearing a reference note first. It runs in families and is far more common in people who started music lessons very young.
About 1 in 10,000 people
Brain Involuntary
Phantom phone vibrations
You feel your phone buzz in your pocket, check it, and nothing happened. Most phone users get this. It is a normal learning effect in the brain, not a fault.
68–89% of phone users
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 Reflex
Photic sneeze reflex
About one person in four sneezes when they step into bright sunlight. It runs in families, and it is probably a wiring mix-up between the nerve for the eyes and the nerve for the nose.
About 18–35% of people
Nerves Perception
Pinocchio illusion
Vibrate the biceps tendon while a person touches their own nose with their eyes closed, and their nose feels as if it is growing. It shows how the brain works out where the body parts are.
Most people, in lab conditions
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 Variation
Plantaris muscle
A small calf muscle with a very long, thin tendon that medical students mistake for a nerve. Between 7% and 20% of people do not have it.
Missing in 7–20% of people
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%
Ears Variation
Preauricular pit
A tiny hole in the skin just in front of the ear, present from birth, that leads to a short blind tunnel. It is harmless unless it gets infected.
0.1–10% of people
Nerves Involuntary
Precordial catch syndrome
A sudden, sharp, stabbing pain near the heart that lasts seconds to a few minutes and gets worse when you breathe in. It is the most common cause of chest pain in children and teenagers, and it is harmless.
Most chest pain in ages 6–20
Brain Perception
Prisoner's cinema
After hours or days in darkness, people start to see light shows: colours, geometric patterns, and sometimes figures or scenes. Prisoners, miners, polar explorers, and meditators all report it.
Up to 77% of people after 4 days blindfolded
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
Ears Sound
Pulsatile tinnitus
A whooshing or thumping in one ear that keeps time with your pulse. It is the sound of blood flowing near the ear. It is usually harmless, but it always deserves a check-up.
About 3–5% of people with tinnitus
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 images
A candle held near someone's eye produces four tiny reflections, one from each surface of the cornea and lens. Eye trackers and eye doctors use them. In the right conditions you can see one of them inside your own eye.
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%
Nerves Involuntary
Random shivers
A single brief shudder that runs down the spine or through the whole body when you are not cold. It is very common, and nobody has studied it formally.
Very common, no formal count
Nerves Reflex
Referred pain
Pain from an internal organ often shows up somewhere else, like the left arm during a heart attack or the shoulder from a gallbladder. The nerves from the organ and from that patch of skin share a line into the spinal cord.
Everyone
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 Rare ability
Savant syndrome
A very small number of people with a developmental disability or brain injury have one extraordinary skill, such as instant calendar calculation, drawing a city from memory, or playing a piece of music after one hearing.
About 1 in 1,000,000 people
Heart Involuntary
Seeing stars when standing up
You stand up fast, your vision goes grey or sparkly, and your head feels light for a few seconds. Blood has briefly pooled in your legs. It passes on its own.
Almost everyone, at some time
Brain Rare ability
Short sleeper syndrome
A small number of people need only 4 to 6 hours of sleep a night, wake without an alarm, and feel fine. Several gene variants that cause it have been found. Most people who sleep that little are simply sleep-deprived.
Well under 1% of people
Body Variation
Situs inversus
All the organs in the chest and belly sit on the opposite side from usual, as in a mirror. About 1 in 10,000 people are built this way, and most are healthy.
About 1 in 10,000 people
Brain Involuntary
Sleep paralysis
You wake up, or start to fall asleep, and cannot move or speak for a few seconds to minutes. Sometimes a threatening presence seems to be in the room. It is the normal paralysis of dreaming sleep running a little late.
8–30% of people, lifetime
Organs Sound
Stomach growling
The rumble from your belly is gas and liquid being pushed through your gut by waves of muscle. It is louder when you are hungry because there is less food to muffle it.
Everyone
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
Skin Variation
Supernumerary nipples
An extra nipple, usually small and flat, on a line running from the armpit to the groin. Between 1 in 500 and 1 in 18 people have one, and most think it is a mole.
0.2–5.6% of people
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
Brain Perception
Synesthesia
In some people, one sense triggers another. Letters have colours, sounds have shapes, or days of the week sit in fixed places in space. The pairings are automatic and stay the same for life.
About 4% of people
Ears Sound
Tensor tympani thump
The low thump or rumble in your ears when you yawn, cough, or shout is a tiny middle ear muscle tensing to protect your hearing from your own noise.
Everyone
Eyes Perception
Tetrachromacy
Most people have three kinds of colour-sensing cone. A few people have four. In theory they can tell apart colours that look identical to everyone else. Confirmed cases are extremely rare, and online tests cannot detect it.
A handful of confirmed cases
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
Ears Sound
The sound of silence
In a room that absorbs almost all sound, you hear your heartbeat, your blood, your breath, and a high ringing from your own inner ear. True silence is not possible for a living person.
Everyone, given a quiet enough room
Ears Involuntary
Tinnitus
A ringing, hissing, buzzing, or roaring sound with no source outside your head. Between 15% and 20% of people have it. It is mostly the hearing system filling in for damaged or tired inner-ear cells.
About 15–20% of people
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 Variation
Tongue tie
A short or tight band of tissue under the tongue that holds the tip down, present in about 1 in 20 babies. Some cannot breastfeed well, but many grow up with no trouble at all.
About 5% of babies
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%
Mouth Involuntary
Tonsil stones
Small white or yellow lumps that form in the folds of the tonsils and sometimes come out when you cough. They smell bad and feel like something is stuck in the throat. They are harmless.
About 10% of people; up to 46% on CT scans Icky
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
Heart Reflex
Vasovagal syncope
The most common reason people faint. A trigger like blood, pain, heat, or standing too long makes the vagus nerve slow the heart and drop blood pressure. The brain briefly runs short of blood and you go down.
About 4 in 10 people faint at least once
Brain Involuntary
Visual snow
Tiny flickering dots across the whole field of vision, like static on an old TV, present with eyes open or closed. About 2–3% of people have it. The eyes are fine. The visual part of the brain is over-active.
About 2–3% of people
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
Bones Trick
Voluntary joint dislocation
Pushing a joint partly or fully out of its socket on purpose, then back in. About 20% of people have some joint hypermobility. True voluntary dislocation is much rarer and often linked to a connective tissue disorder.
Rare; no formal count Icky
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
Bones Variation
Webbed toes
Two toes joined by skin, most often the second and third, in about 1 in 2,000 to 3,000 babies. The toes usually work normally and many people leave them as they are.
About 1 in 2,000–3,000 births
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