As Dr. Aaron E. Carroll explains,
- Poinsettias aren't actually poisonous
- None of those miracle hangover cures really works
- Suicides do not increase around the holidays
- Most body heat is not lost through the head
- Eating at night doesn't make you fat
Here's a link to an article on these myths in a medical journal. (Access requires a subscription or signing up for a free trial): http://www.bmj.com/content/337/bmj.a2769
And while we're at it...
Turkey doesn't make you sleepy: http://youtu.be/vIK8PR6tzBU
And sugar doesn't make kids hyper: http://youtu.be/mkr9YsmrPAI
The last one in particular is an excellent example of something called "confirmation bias": a natural tendency to look for confirming examples and disregard contradictory ones. Kids like sugar, kids tend to have a lot of energy, so if you spend any time around kids, it's easy to think of plenty of examples of kids going wild after eating sugar. Moreover, since sugar is, loosely speaking, food energy, the connection sort of makes sense. Moreover, parents tell kids that they go wild after eating sugar, so they do it, even if only a version of the placebo effect.
So unless you do a careful study recording not only when kids go wild after eating sugar but when they go wild without sugar or consume sugar without going wild, and ideally when you have some way to keep the kids themselves from knowing they're eating sugar, it's easy to be misled.