Being cold and wet does not cause colds. You also won't catch a cold from going outside without your coat (another warning your mother may have issued when you were young), although you'll probably feel very chilly. And you won't catch cold from going to bed with a wet head -- even if the air conditioner is running at full blast (though you might wake up with a pretty funky hairdo).
Colds are actually caused by viruses. You need to be exposed to the cold virus in order to get sick. More than 200 different viruses can cause colds, but the biggest culprit is the rhinovirus [source: NIAID].
When someone sneezes or coughs and isn't kind enough to cover his or her mouth and nose, droplets containing the cold virus escape out into the air. If you're close enough to some of these droplets, there's a good chance you'll soon be coming down with a cold. Viruses also can live on sinks, counters and other surfaces, which means you can catch a cold if you touch an object that was recently handled by someone with a cold, and then put your hands on your nose or mouth.
Going outside with a wet head in winter won't make you sick, but cold weather can make you more susceptible to catching a bug. However, it's not the temperature, but the humidity (or lack thereof) that's to blame. Scientists have shown that cold winter air (which is less humid than warm summer air) can dry out the mucus lining of your nasal passages, making it easier for viruses to get in and make you sick.
Kids are also more likely to catch colds during the fall and winter, simply because that's when they're at school. Classrooms are close quarters, which make them breeding grounds for all sorts of germs.
Read about more debunked myths on the next page.