When I was younger, having never actually seen a male foreskin, I believed that circumcision involved the removal of part of the glans of the penis (i.e. the little bit on the end that makes it look like a mushroom).
Thus I believed that I was uncircumcized until a Jewish friend disabused me of the notion by telling me his looked the same as mine. Little kids think weird things, right?
We were in 9th grade.
A childhood belief about rude bits

others believed things like this
- I grew up Jewish. Until I was about 10, I believed that everyone who wasn't Jewish was Catholic.— Philm35
- My friend's sister believed the lyrics to Gazza's "Fog on the Tyne" were "Fog on the dinasaurs mine all mine"— Suzanne
- a male friend of mine used to think that only women who had been pregnant had periods— oiman
- I used to believe that gay sex involved a man sticking his penis inside another man's penis.Anon
- I once told a friend of mine that if you held in a smelly burp, it'd turn into a fart. She believed me.Anon
- On this site when people sign "Anon" I thought Anon was one person who believed in a lot of weird things...— Slow to catch on.