When I was a kid I heard on the news that Tom Green only had one ball. (I found out much later this was because of cancer.)
At the time I assumed that the number of testicles a boy had varied. I guessed that they most commonly had 1-3. I believed this until about seventh grade.
A childhood belief about rude bits

others believed things like this
- when i was little i believed that your social security number was the number you were born on the earth — christine
- When i heard about Irish Bombers on the news during the 70s i used to think that planes were dropping bombs.— Moose
- Mom told us that if we didn't eat every single bite of our veggies at dinner time, we'd get colon cancer.Anon
- I believed you had to color all the dinosaurs in coloring books green because all the dinosaurs WERE green.— Betty
- I used to think volleyball was actually called "ball-y ball"Anon
- When I was a kid I believed that the number 11 was pronounced "onety-one"— Alicia