"Don't cross your eyes - they'll get stuck that way!" You probably heard this as a child, but then figured out on your own - when your eyes didn't stay crossed - that it wasn't true. Some old wives' tales, though, aren't as easy to debunk, because they are repeated so often and believed by so many. Ask yourself these true-false questions to see if you can separate medical fact from medical myth.

1. Poison ivy is contagious.
   False. The only way to get the rash is from contact with the oil (urushiol) of the poison ivy plant.As long as urushiol has been washed off, you can't catch poison ivy from exposure to the rash.
2. Reading in dim light or sitting too close to the TV damages your eyes.
   False. Reading in dim light may cause eye strain, but the symptoms are temporary. You can, however, damage your eyes by looking into a very bright light, like the sun.
3. If you pick it up within five seconds ("the 5-second rule"), it's safe to eat food you dropped on the floor.
   False. Whether you pick up dropped food within a nanosecond or wait until the next day, you can be fairly sure you're picking up bacteria with it.
4. Swallowed gum is harmful to the digestive system.
   False. Contrary to what we've all heard, gum doesn't stay in our stomachs for seven years. Unless you swallow it in huge amounts, gum passes swiftly through the digestive system. Because the body can't break it down, though, it exits the body looking pretty much the same as when it entered.
5. Placing cucumbers or tea bags on your face helps reduce wrinkles.
   True. Botanical elements in cucumbers and tea can temporarily smooth out wrinkles.
6. You lose 50 percent of your body heat through your head, so always wear a hat.
   False. The real number is about 10 percent, which is roughly the amount of your body's surface that would be unexposed. Any part of the body that is uncovered will release a lot of heat.
7. You can get pregnant when you have your period.
   True. Not every woman ovulates precisely in the middle of her cycle. A woman who has sex during her period may still have live sperm in her reproductive tract when ovulation occurs. Birth control should still be used during menstruation.
8. We use only 10 percent of our brains.
   False. Scans of the brain reveal 100 percent activity. In the course of a day, every part of the brain is used.
9. You should wait one hour after eating before swimming.
   False ... and true. When your kids want to jump in the pool after a meal to splash and play, let them. But if you're an avid swimmer who is planning some serious laps, waiting an hour is a good idea. That gives your food time to digest. Otherwise, the blood flow to your muscles can decrease, possibly causing cramping.
10. Teething causes fever, runny nose and diarrhea.
    False. These symptoms may appear during teething, but aren't caused by it. Talk to your child's doctor about your baby's symptoms.
11. Your heart stops beating whenever you sneeze.
    False. Although your heart's rhythm may change for a very short time, your heart never stops beating.
12. Eating red meat is bad for your health and makes it hard to lose weight.
    False: Small portions of lean red meat, along with lean pork, chicken and fish, can be part of a healthy diet. Lean meat also contains nutrients like iron, protein and zinc.
13. If you want to lose weight, you should not eat after 8 p.m.
    False: It doesn't matter what time of day you eat. What matters is your total intake of calories and the amount of physical activity you get.
14. Dementia is a normal part of aging.
    False: Dementia is a condition, not a natural result of aging. It can be the result of a medical condition, such as high blood pressure, diabetes or obesity. It can also be a side effect of medication or poor nutrition.

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External Sources University of Arkansas for Medical Sciences. Medical myths. . Accessed September 16, 2009 National Institute of Diabetes and Digestive and Kidney Diseases. Weight loss and nutrition myths: how much do you really know?. Accessed September 25, 2009 University of Illinois, College of Agricultural, Consumer and Environmental Sciences. If you drop it, should you eat it? Scientists weigh in on the 5-second rule. . Accessed September 09, 2009 University of Arkansas for Medical Sciences. Medical myths; Do you have to wait 30 minutes after eating before swimming?. Accessed September 25, 2009 Vreeman RC. Mixed messages. Medical myths. British Medical Journal. 2007;335(7663):1288-1289. Planned Parenthood. You can't get pregnant when you have your period. . Accessed September 25, 2009

This article was reviewed and updated 08-01-2009.