What's the biggest myth, you were taught in school as being fact?

Reading some of the threads on the new Cosmos series got me to wondering about it when someone stated that people in the middle ages thought the Earth was flat, as a whole. When in fact, it was common knowledge it was a globe, just much different in size than most thought and much of it unexplored.

What's some other things you've found to have been myth, or flat out wrong as you've matured and learned more in life?