And the mantra of the health and beauty world, "eight by eight," which means you should drink eight 8 oz. glasses of water every day? Lots of people believe it. Some schools require kids to carry bottled water around with them. But it's another myth.
Dr. Heinz Valtin, professor emeritus of the Dartmouth Medical School, spent his life studying the right balance of water in our bodies, so there's no evidence that supports the "8 x 8" idea.
"I drink about five or six glasses per day -- only one of them is water," he said.
Much of the fluid we need comes from, of all things, food.
"Even a slice of white bread is more than 30 percent water," he said. "It's lots of water, 80 to 90 percent in vegetables and fruits."
Valtin acknowledges that drinking water is not a bad idea. "What's wrong with the myth is that the recommendation is universal that every last one of us, including, as one article said, couch potatoes, must drink at least eight, 8-ounce glasses per day," he said.
The Institute of Medicine's food and nutrition board agrees with Valtin. It says drinking eight glasses of water is not necessary, because we get plenty of fluid from our food. When your body does need more fluid, it has a marvelous mechanism for telling you to drink up. It's called "thirst."