When I was about 7 or 8 years old, my next-door neighbor (about 13 at the time) tried to explain sex to me. Unfortunately, he gave me a very poorly worded description of a hymen by saying, "when a girl first has sex, a bone in her vagina breaks, and it hurts really bad." Of course, my inexperienced mind immediately assumed there was a cage of bones, like a ribcage, inside every woman's vagina. I had trouble understanding this anyway, so when I learned the truth several years later, it was a relief :) .
A childhood belief about rude bits

others believed things like this
- When I was a little girl, I thought all babies came from 'China' (a Vagina)— Lainie
- i used to believe that pee came out of the vagina— Janey
- i used to believe that a penis was the same as a vagina except really long— *stupid*
- I don't know why or where I got this from, but when I was little, I referred to my vagina as my "goose". — StrangeLilGrrl
- i used to believe that if you have sex for too long, the penis would stay in your vagina— Angela
- Until I was ten or eleven I thought a vagina was a "pagina" Anon