I used to think that PMS stood for "Pre Marital Sex", and was always confused as to why the girls in my 7th grade PE class had it every month.
A childhood belief about periods

others believed things like this
- In my second grade class everyone thought that sexual intercourse was "TALKING DIRRTY".— Israel Bliss
- I literally thought “oral sex” meant talking about sex until 8th grade health class.— Kai
- In first grade a boy told me that he could see girls naked with his microscope lens. I beleived it for years.— Jeanette
- i used to believe (because my dad told me so) that AM stood for "awful music" and FM stood for "fine music"— skydvgrl
- I used to believe that the Chinese boy in my kindergarten class flew from China every day to get to class.— Debi
- I thought that the difference between girls and boys was that girls have teeth.
Anon