I have a sister 3 years older than me. As a small child my parents stopped me from 'playing with myself' by telling me that "If you do that too much it will drop off. Look what happened to your sister!", for years I thought that all girls were ex boys that couldn't stop themselves.
A childhood belief about rude bits

others believed things like this
- My oldest sister is 15 years my senior. I used to think that she was older than my mom.— Callie
- when i was really young, i couldn't figure out why i can't get older than my older sister.— angela
- When my mom was a child, she thought that while girls had periods, boys had hemorrhoids!Anon
- a friend of my little sister's had 4 older brothers, so she thought that when you got older you grew a penis!Anon
- I thought that 1983 went on for 3 years - had to ask my sister who decided how long a year lasted.— Sally
- I thought if 3 cards stopped at a 3-way stop sign then they had to stop forever until Christmas.Anon