When I was a young boy - around 9 - I remember announcing seriously one day at the dinner table that when I grew up I would only be able to have 2 children. My mother asked me why and I replied "Because I only have 2 testicles".
I was not prepared for my parents reaction. They literally fell about laughing to the point where they were literally wiping tears from their eyes. I just sat there saying "What? What?"
Of course, I eventually got the explanation but it wasn't until some time later that I too saw the funny side.
A childhood belief about reproduction

others believed things like this
- When my mom would say that my eyes were bigger than my stomach I thought she meant it literally.— Amanda
- I used to believe that all my energy "came from my nipples and my back" I literally said that to my parents.— Eleanor R
- that if you had fun, time would literally pass by faster— paul
- i used to watch doctor who and every time i saw a darlic i wet myself literally.— Dan Barber
- that when I grew to be an adult my parents would be children.— Rainbow
- I used to confuse tentacles and testicles when I was younger. It was funny— Chelsea