As a child I knew that girls didn't have penises, but I thought they might have testicles.
I also thought that women's breasts contained their lungs.
A childhood belief about rude bits

others believed things like this
- I used to believe both boys and girls had penises so i thought sex was when they tied them together.Anon
- I used to think that women had penises, but that they were just hairier than mens'.— Ameeratron
- When I was 5 I thought that baby boys came from men and baby girls came from women...— Gaz
- I thought pads sucked blood from women. It wasn't until later in life that I knew their actual purposeAnon
- When my mom was a child, she thought that while girls had periods, boys had hemorrhoids!Anon
- I used to think that girls had "balls" and not boys, and the "balls" were slang for breastsAnon