I thought that only men had hair under their arms, and that women didn't. Kind of like how only men can grow beards- same thing. I was a teenager when my mom told me that women have armpit hair- they just shave it off. It took me awhile to believe it. I'm a male, by the way.
A childhood belief about hair

others believed things like this
- i used to believe that if you shave your head you could grow any type of hair like curlyAnon
- I thought it was illegal for women to use Just For Men hair coloring— Brainhead
- I believed that if you ate a piece of your own hair, a new strand of hair would grow on your head.— Ryo
- I used to think that everybody's hair was black and if you had blond hair, you dyed your hair.— ~_~CoolKid93_~_
- My father always told me that if I wore socks to bed I would grow hair inbetween my toes. Anon
- When I was little I thought that dying your hair means you kill your hair.Anon