The tiny muscles at the bottom of hair follicles contract in response to cold, fear or other emotions. In furry mammals, this causes hairs to “stand on end,” trapping a layer of air as insulation and making the animal appear larger to scare off predators. In humans, it is a “vestigial reflex” (it no longer serves a purpose) and instead just makes skin look temporarily rough and pimply.
Very cold foods can overstimulate the nerves on the roof of the mouth. The sudden stabbing pain in your head, also known as “brain freeze,” is an example of referred pain. To avoid it, eat ice-cold foods slowly and try to keep them away from your palate. To counter it, press your tongue to your palate or drink something lukewarm.
The popping sound may come from the rapid release of gas bubbles that form in a joint when it is stretched. It can occur when you bend your neck or wrist, or twist your back, for instance. It is usually harmless, but if you’ve had an injury or if the cracking is accompanied by pain or swelling, see your doctor. It could be a sign of arthritis or other joint problems.
The photic sneeze reflex, also called sun sneezing or the “ACHOO syndrome” (which stands for autosomal-dominant compelling helio-ophthalmic outburst), occurs in about one in four people when they are exposed to sudden bright light. It runs in families, and most people sneeze two or three times. Scientists blame it on a crossover of nerve signals—when bright light overstimulates the optic nerve in the eye, it “accidentally” also stimulates the nerve responsible for the sneeze reflex.
Involuntary twitching of the eyelid muscles may be caused by fatigue, stress, eye strain or possibly caffeine. It may occur on and off for several days. If the contractions are severe enough to close your eyelid completely, involve other muscles in the face, are accompanied by other eye symptoms or don’t stop after a week, see your doctor.
No one knows for sure why people yawn. One theory is that yawning increases oxygen intake when needed, but studies have shown that people yawn, usually when they are tired or bored, even if they have high blood oxygen levels. It may be a protective reflex that helps maintain proper lung function. Yawning is also contagious: seeing someone yawn—or just hearing, reading about or even thinking about yawning—makes some people yawn. This may have served as a primitive form of social bonding, and still seems to synchronize sleep in groups of animals.