When I was little, I thought that if a child had a period of growing quickly, it was called a "growth sperm." Well, one of my friends came over to my house to play, and my mom mentioned to my friend that she has grown a lot since the last time she had seen her. So I blurted out, "YEAH, YOU'VE HAD A GROWTH SPERM!!!". Obviously, I later learned that it's called a growth SPURT, not sperm.
A childhood belief about getting older

others believed things like this
- There was a Chinese takeaway near us called the Orient Express. I thought it was Onion Press. Yeah.— Spook
- I used to think that adult kangaroos were called Kangas and that child kangaroos were called Roos. Anon
- I use to think that Bruce Springsteen's song "growing up" was called "throwing up"— glowworm
- When I was little - before we learned cursive - my sister and I called it 'real writing'. — Monty
- My friend thought butterfly was actually called a 'flutterby.'— BlutterBly?
- I believed that the breed of dog called Boxer was called that because they could stand up and box.Anon