I used to think the word "sane" was slang for "insane" so if one of my friends was bouncing off the walls, I'd tell them, "You're going 'sane!"
A childhood belief about speaking

others believed things like this
- Bouncing on my bed and watching houses through the window I was sure they were literally bouncing with me.Anon
- A motorcycle was started by jumping up and down and bouncing on the seat.Anon
- I used to think Cockneys only ever talked in rhyming slang.Anon
- "Chassis" is slang for a woman's body, especially her genitalia.
— Siddharth Mehrotra - For whatever reason, I thought that grass was slang for tobacco rather than marijuana.Anon
- i used to think the word "friendship" meant a special kind of rocketship that you went on with your friends.— chips ahoy!