I thought that people were filled with air and that if you "untied" your bellybutton you would deflate and fly around the room(like a ballon does) or pop.
A childhood belief about belly buttons

others believed things like this
- My older brother used to tell me that if I "untied" it, I'd fly around the room like a balloon. — Gullible youth
- i used to believe that people's buttocks were plump because of the poo filled in it— Diesa
- I used to believe that as I ate food, I filled up, and that once I filled up completely, I would die. Anon
- I thought liposuction was squeezing the fat out of your bellybutton. Too bad it's not that easy...— Angela
- If you touch your bellybutton, it will get infected.Anon
- That if you fiddled with your bellybutton it would unscrew your bum and it would fall off— Sundaeg1rl