As a little boy, I knew that girls and boys had different parts between their legs, but I thought that a girl's parts were the same as a boy's parts, just pushed inside (inverted). Made sense to me.
How did they get that way? Well, at first, boys' and girls' parts were the same, then a dinosaur kicked a girl in the crotch....
A childhood belief about rude bits

others believed things like this
- When I was little I thought girls had "man parts" as well, but it just took longer for them to grow.— person
- When I was a little girl, I believed that all cats were girls and all dogs were boys. :)— Mary Ellen
I used to believe a man and a woman needed a doctor to put their parts together when they did it.— Phil- When I was little I believed that boogers were parts of my brain that got used up.Anon
- i used to believe that boggies are parts of fluff that fly in to your nose— natasha hawthorne
- When I was a child, I used to believe that armpits were private parts— Duda Gueiros