Well, this isn't mine, but I had both of my daughters convinced that I could "take" their belly buttons and give them back. The older one was 4 or 5 when she finally figured out that I was fooling.
A childhood belief about belly buttons

others believed things like this
- i uses to believe that boys wee'd out of their belly buttons.— boys and toilets
- I used to believe that babies came out of belly buttons — kels
- that bunnies came out of belly buttonsAnon
- I used to belief that women had babies by pressing their belly buttons!— amy
- i used to believe that guys pulled out their outie belly buttons and that's how they peed.— Sky
- I used to believe that belly buttons were for putting the salt in while you were eating celery in bed.— David T