What are some urban legends of physiology?

Here's one weird one. We close our eyes when we sneeze to keep them from popping out of our heads. Similarly, we sneeze with our eyes closed to keep the stuff we sneeze out from getting in our eyes. The real reason we close our eyes while sneezing is due to reflexes.

Another is how taste is mapped on the tongue into four areas - sweet, sour, salty and bitter. There are five basic tastes identified so far, and the entire tongue can sense all of these tastes more or less equally.

There's also the very prevalent myth that hair grows faster if you cut it. This is not true; hair is dead and it has no way of "signaling" the hair follicle if the end of it is cut off, but this myth is due to the fact that hair looks thicker and fuller when dead split ends are cut off.

Another one is that sugar makes kids hyper, although that's debatable. Sugar does not make kids hyper like, say, caffeine would, but it is a source of energy just like any other food. Many foods that aren't very sweet do contain a lot of sugar, like bread, but no one claims that bread makes kids hyper.

I heard a new one just the other day - every 10 times you rub your eye produces one wrinkle.

Cracking your knuckles brings on arthritis.

Swallowed bubblegum accumulates in your appendix and causes appendicitis.

When you hiccup your heart skips a beat.

Can't think of anymore right now! :-)

good question