I believed that periods were one-time spurts of blood. After I got mine and saw that I was still bleeding later, I assumed I must have hurt myself somehow and didn't tell my mom.
(I wish they only lasted for that one second...)
A childhood belief about periods

others believed things like this
- a male friend of mine used to think that only women who had been pregnant had periods— oiman
- My friend's sister believed the lyrics to Gazza's "Fog on the Tyne" were "Fog on the dinasaurs mine all mine"— Suzanne
- I also used to think that Mommies didn't have periods. I thought it went away once you had a baby.— T
- when I was young, I heard that blood had iron in it. I thought blood was red because it was rusty.— Hunter
- One kid I know believed that because I wear glasses, it wouldn't hurt if he poked my eye.Anon
- I believed that all of my blood was stored in the calves of my legs. — Julia