Why Do We Sneeze?

Cold air causes blood vessels in the nose to dilate and mucus production to increase as part of the nose's job of warming and humidifying inhaled air before it reaches the lungs, and cold temperatures also slow the tiny hair-like structures that normally move mucus backward, causing it to pool and drip forward instead. One of the nose's core functions is conditioning the air you breathe, warming it to body temperature and adding moisture before it reaches the sensitive tissue of the lungs. When cold, dry air enters the nasal passages, blood vessels in the nasal lining dilate, increasing blood flow to help warm the incoming air, and mucus glands increase production to add humidity. At the same time, cold temperatures slow the normal function of cilia, tiny hair-like structures that continuously sweep mucus backward toward the throat to be swallowed. With cilia moving more sluggishly in the cold, mucus accumulates faster than it can be cleared through its usual pathway, and gravity pulls the excess forward and out the front of the nose instead, producing the familiar cold-weather runny nose.

Yawning is weird because scientists still don't have one clean answer. The old idea, that your body needs more oxygen, has been mostly ruled out. What seems more likely is that yawning happens when your brain is shifting gears: moving from low activity to high, or the other way around. It may also help cool a warming brain by pulling in cooler air and increasing blood flow. It happens when you're tired, waking up, bored, or stressed. Every vertebrate animal does it, which tells us it has been around a very long time and probably matters, but exactly why is still being worked out.

Your fingers are not just soaking up water like pasta in a pot. The wrinkling is an active response controlled by your nervous system. When your hands are submerged for a few minutes, your sympathetic nervous system sends a signal to the blood vessels in your fingertips. They constrict, narrow down, which reduces the volume inside your fingertips. The skin, with nowhere to go, folds into ridges. The proof: if the nerve to that finger is cut, the finger stops wrinkling in water entirely. It's your body responding to the wet environment, possibly to improve grip on wet surfaces.