When I was little, don't remember exactly the age but it went on for quite a while, I used to believe that the more frequently you had sex, the bigger your breasts or penis would become. So somebody with big breasts or a big penis obviously had sex every day, and someone with smaller breasts or a smaller penis didn't have sex very often. I'm not really sure when I stopped believing this.
A childhood belief about rude bits

others believed things like this
- i used to beleive when you drank milk your breasts would get bigger so i used to look down at them afterwordsAnon
- I used to think that Frankie Avalon sang, "Hey Penis, Oh, penis..." instead of "Hey Venus."Anon
- I used to believe that gay sex involved a man sticking his penis inside another man's penis.Anon
- I used to believe that gay guys had sex by sticking their penis in the other's penis.— Fanbrat
- I used to believe that babies came from between/underneath the breasts.Anon
- I used to think that the shoulder blades were actually "back breasts".— Kdees