I heard the joke "how do you get down from an elephant? You don't. You get down from a duck"
I didn't know that "down" had two meanings. So, for the longest while, I thought that if someone were sitting on an elephant, that they couldn't dismount directly but had to use a two-step process. First, move from the elephant to a duck. Then get off the duck.
A childhood belief about general

others believed things like this
- i thought that "aromatic" duck was actually "asthmatic" duck.
poor thing...— rachael - I always thought that duck sauce (from chinese restaurants) was duck poop.
— emily - Whenever I was upstairs on a bus, I didn't believe that it would fit under the bridge so I used to duck. — Dave, Lancashire
- I used to belive that if I didn't duck when going under bridges they would hit me.Anon
- I wanted to be an elephant when I grew up. — Vix
- I used to believe that salami was the meat from elephant's feet— Sarah