I used to believe that when the egg was fertilized, it would become a baby in about a second, pop out of the mom's stomach, and fly around the room like a balloon. I have no idea where that last part comes from.
A childhood belief about reproduction

others believed things like this
- If you swallowed chewing gum it would make a balloon inside you and your stomach would explodeAnon
- 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 beleive that my stomach growled when I was hungry because I was part tiger.— Matthew
- When i was young I used to believe if I ate ham fat I was going to inflate like a balloon— a guy
- i usd to believe that if i put a balloon on a lawn chair that i could float up in the sky and around the worldAnon
- up until puberty I thought that my nipple would inflate like a balloon to form a breast— EiEi