Involves a world record or an extreme documented case.
7 oddities.
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.
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.
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.
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.
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.
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.
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.