When I was very young I knew that only boys had penises. That was the extent of my knowledge though as I thought girls weed out of their bums ;)
A childhood belief about weeing and pooing

others believed things like this
- I thought that the difference between girls and boys was that girls have teeth.
Anon - I never knew why boys had to lift up the toilet seat...until my friend told me that boys had bad aim.Anon
- I always thought when I was a kid that Milo is for boys and that milk are for girls. . . — Cathyia
- i thought cats were girls and dogs were boys. Anon
- I always thought cats were all girls & dogs were all boys ... I have no idea why.Anon
- I used to believe that little boys grew up to be girls and little girls grew up to be boys. — Darin