When I was very young, I used to believe that when you went to the trash room (we lived in a high-rise apartment house) and the sign on the door said "REFUSE," it meant that the room was all full of trash and there wasn't space for any more.
Well, my mom asked me to take out the trash one day. When I returned to the apartment with the bag of trash still in my hand, she looked puzzled and asked me why I didn't put the trash out.
I took her with me to the trash room, pointed to the sign on the door, and said, "See? It says REFUSE. It's all full! It refuses to take any more!"
She then explained (after laughing hysterically) that the word "REFUSE" had two meanings -- and one of them meant trash!
Duuuuuuuhhhhhhh...
Did I mention that I'm now a professional writer/editor???
A childhood belief about around the house

others believed things like this
- When I was young I want to grow up yo be a trash man, becuse they only work one day a week.— Steve H
- My 5-year old cousin used to believe the garbagemen were stealing their trash.Anon
- I thought for the longest time that a No Standing traffic sign meant no person should stand near that sign.— Shannon
- Before I went to a planetarium for the first time, I thought it was just a room full of plants. Anon
- I used to believe you got high tides when the sea was full (I lived on the Thames, then) — rosy
- I thought that the sign "!" meant: "look at the wonderful landscape"— Giorgos