All of the girls in my family were told that we had to keep our chests warm so breasts would grow.
I don't know where it came from, but I kept them warm and have larger ones while my sister did not and is flat chested.
A childhood belief about rude bits

others believed things like this
- I used to believe that you could keep a chicken egg (that you bought) warm, and it would hatch!Anon
- I use to believe that everyone in Florida didn't go to school because it was warm all year around.— Julia
- When I was little I thought people smoked cigerates to keep warm.Anon
- I used to believe that I could make a chick hatch from egg carton as long as I kept it warm :)— Stephanie
- I used to think condoms were what males wore to keep their "privates" warm during winter.Anon
- When I was young I thought people smoked to stay warm.— Phil Stam