My girlfriend and I both have black hair. When she was making a crude joke, I realised that she thought people's pubic hair matched the colour of the hair on their head! We were in our early twenties at the time.
I guess it's perfectly logical, as people's body hair varies in colour, and she just hadn't seen the pubic hair of anyone who didn't have black hair.
I gave her a hard time about it for a few weeks, though.
A childhood belief about rude bits

others believed things like this
- I used to think pubic hair was really called 'public hair' until I was about 15— Spe
- I used to think that everybody's hair was black and if you had blond hair, you dyed your hair.— ~_~CoolKid93_~_
- I believed that if you ate a piece of your own hair, a new strand of hair would grow on your head.— Ryo
- When I was little I thought that dying your hair means you kill your hair.Anon
- I used to hold my hand over my hair whenever i left the house and it was windy incase my hair blew away— lisa
- I used to believe that you could curl your hair by twirling a strand of hair wrapped around your finger— Anonymous