This one is actually my sister's. When she was about 4 she asked my mom what the belly button was for. My mom told her it was connected to your butt and it held it up by a string.
Well, years went by and my sister was in middle school. She had to take the requisite sex-ed class and the teacher asked if anyone knew what the belly button was for. My sister's hand shot up and she said, "I know, I know. It holds your butt up!" Everyone in the class started laughing at her and she was mortified, and to this day (she is now 38) I think she is still upset with my mom about it.
A childhood belief about belly buttons

others believed things like this
- My younger sister used to believe that in everybody's belly button there lived a family of squirrels.— Sophie
- My dad told me when I was little that my belly button was "Where the Indian shot me"Anon
- i used to believe that if you poked yerr belly button yerr butt would fall off.— jordy
- I used to think that my belly button was where I was once shot and that the navel was a healed up bullet hole— Tony
- I used to believe your belly button was a knot that held you together - my parents had wierd anatomical views— anne
- I used to think that if i touched my belly button it would unfold and my belly would be inside out forever. Anon