Up until the late 16th century, everyone "knew" that the sun and planets revolved around the Earth. Up until the late 19th century, epidemic illnesses such as cholera and the plague were "known" to be caused by a poisonous mist filled with particles from rotting things. Up until the early 20th century, the most common procedure performed by surgeons for thousands of years was bloodletting, because we "knew" that blood drained from the body balanced the whacky humors responsible for poor health. Well alrighty then.

But as misinformed as all that may sound now, our predecessors believed these "facts" with the same certainty that we believe that the Earth is round and hot fudge sundaes make us fat.

Living in a time of such dazzling science and technology, we stand firmly behind our beliefs ... even if so much of what we think we know to be correct is actually wrong. Here are some of the more common misconceptions, ideas that may have started as wives' tales or that came from a faulty study that was later proven wrong. Whatever the case may be, these facts are false.

1. Going out in the cold with a wet head will make you sick

"Put a hat on or you’ll catch your death of a cold," screeches every micromanaging momma as her charges march off into the winter wonderland. But in numerous studies addressing the topic, people who are chilled are no more likely to get sick than those who were not. And a wet or dry head makes no difference. (But these tips can help you stop a cold before it starts.)

2. Vikings wore horned helmets

Is there anything more "Viking warrior" than a helmet fitted with horns? Nary a portrayal shows the seafaring Norse pirates without the iconic headgear. Alas, horned hats were not worn by the warriors. Although the style did exist in the region, they were only used for early ceremonial purposes and had largely faded out by the time of the Vikings. Several major misidentifications got the myth rolling, and by the time costume designers for Wagner’s "Der Ring des Nibelungen" put horned helmets on the singers in the late 19th century, there was no going back.

3. Sugar makes kids go bonkers

The Journal of the American Medical Association published a review of 23 studies on the subject of kids and sugar, the conclusion: Sugar doesn’t affect behavior. And it's possible that it is the idea itself that is so ingrained as fact that it affects our perception. Case in point: In one study mothers were told that their sons had consumed a drink with a high sugar content. Although the boys had actually consumed sugar-free drinks, the mothers reported significantly higher levels of hyperactive behavior. That said, some scientists warn that sugar can make you dumb.

4. You lose most of your body heat through your head

Everyone knows that you lose somewhere around 98 percent of your body heat through your head, which is why you have to wear a hat in the cold. Except that you don’t. As reported in The New York Times and elsewhere, the amount of heat released by any part of the body depends mostly on the surface area — on a cold day you would lose more heat through an exposed leg or arm than a bare head.

5. You will get arthritis from cracking your knuckles

It seems reasonable, but it's not true either. You will not get arthritis from cracking your knuckles. There is no evidence of such an association, and in limited studies performed there was no change in occurrence of arthritis between "habitual knuckle crackers" and "non crackers." There have been several reports in medical literature that have linked knuckle cracking with injury of the ligaments surrounding the joint or dislocation of the tendons, but not arthritis.

6. Napoleon was short

Napoleon's height was once commonly given as 5 feet 2 inches, but many historians have now accorded him with extra height. He was 5 feet 2 inches using French units, but when converted into Imperial units, the kind we are accustomed to, he measured almost 5 feet 7 inches inches tall — which was actually slightly taller than average for a man in France at the time.

7. You have to stretch before exercise

Stretching before exercise is the main way to improve performance and avoid injury, everyone stretches ... but researchers have been finding that it actually slows you down. Experts reveal that stretching before a run can result in a 5 percent reduction of efficiency; meanwhile, Italian researchers studying cyclists confirmed that stretching is counterproductive. Furthermore, there has never been sufficient scientific evidence that pre-exercise stretching reduces injury risk.

8. Cholesterol in eggs is bad for the heart

The perceived association between dietary cholesterol and risk for coronary heart disease stems from dietary recommendations proposed in the 1960s that had little scientific evidence, other than the known association between saturated fat and cholesterol and animal studies where cholesterol was fed in amounts far exceeding normal intakes. Since then, study after study has found that dietary cholesterol (the cholesterol found in food) does not negatively raise your body’s cholesterol. It is the consumption of saturated fat that is the demon here. So eat eggs, don’t eat steak.

9. Dogs age at seven years per one human year

Your 3-year-old dog is 21 years old in human years, right? Not according to experts. The general consensus is that dogs mature faster than humans, reaching the equivalent of 21 years in only two, and then aging slows down to more like four human years per year. "Dog Whisperer" Cesar Millan’s site recommends this way to calculate your dog’s human-age equivalent: Subtract two from the age, multiply that by four and add 21.

10. George Washington had wooden teeth

Our first president starting losing his teeth in his 20s, but contrary to popular belief, his dentures were not made of wood. Although built-in toothpicks would have been handy, Washington had four sets of dentures that were made from gold, hippopotamus ivory, lead, and human and animal teeth (horse and donkey teeth were common components in the day). Also of note: The dentures had bolts to hold them together and springs to help them open, all the better to eat one of his favorite confections, Mary Washington's gingerbread.

MNN tease photo of Viking: Shutterstock