A long time ago, when someone would say something about someone "loosing their marbles", I thought that they were refering to their testicles, because I used to call them marbels after my mom said that calling them balls was rude.
A childhood belief about rude bits

others believed things like this
- I originally thought "balls" was slang for buttocks instead of testicles.Anon
- I used to think that girls had "balls" and not boys, and the "balls" were slang for breastsAnon
- i always thought it was Grape Balls of Fire instead of Great Balls of Fire— Courtney
- i used to belive that 100 was the biggest number in the world "i got at least a hundred marbles."— dan harris
- My parents told me I was found under a gooseberry bush which I thought odd for a long long time.— Kate
- When i was little, i thought that when i grew up i could only have two kids because i only had two testicles.— nick