I used to think that "foreskin" was the skin on your forehead. When I was 10, while bathing my 5-year-old sister, I told her to make sure she washed her foreskin really well. I will never live that one down.
A childhood belief about rude bits

others believed things like this
- I used to believe that circuncision was a natural process and that the foreskin fell off during puberty.— Yure
- when i was little my sister told me that when i bite my nails it would cut through my skin and i would die! — nn
- When I was little my dad told me that if you drank too much coffee it would make your skin turn black. Anon
- My sister told me that if you had sweetcorn in your poo, you had six months to live! I cried for weeks — Billy
- When I was a kid I thought all the wrestlers on tv had oranges under their skin to make them look muscular :)— Anonamoose
- In the mid 90's, I used to believe that babies who were born in the year 2000 would live forever.Anon