When I was little, my mom told me that the place I pee out of was my "bottom".
As you might imagine, I got really confused later in life when I heard the word "bottom" used in other ways.
I remember once I was watching a movie, and a mom was talking to her son, and she said something like, "Tommy, if you don't do your homework I'm going to spank your bottom with a belt!"
Not only did I think that was completely AWFUL, I was like, "Boys can have bottoms, too?!"
So for a while I thought boys could have vaginas, and girl's could have penises, and it was only having boobs that determined your sex.
A childhood belief about rude bits

others believed things like this
- I used to believe that your bottom teeth were actually in your bottom— jez
- i thought if you went to the bottom of the earth you would fall off.— Tiffany Amber Stefanko
- I used to believe both boys and girls had penises so i thought sex was when they tied them together.Anon
- I used to believe that when you had cancer, your bottom would be in front instead of in back. Anon
- I used to think that hot dogs were made from boys' penises.— never eating hot dogs again
- I used to believe that elfs lived in the hedge at the bottom of my garden!!
— The milky bargirl