When I was six years old, I persuaded my best friend that if you looked at the full moon (this was a night with said moon) a door would open to another world and you'd end up in a world full of monsters.
He ran home, terrified, with his face pointed firmly at the ground and believed it for years. I have no idea why I said this.
A childhood belief about monsters

others believed things like this
- The moon is full of wotsits.Anon
- I used to believe that the full moon was God's flashlight.Anon
- i used to think that the moon would follow you in the car, also that it looked like a half moon.— natalie k
- I believed for years that the sun and moon were the same thing - it just glowed more in the day :)— Deb
- Until a few years ago, I believed this was a "doggy dog" world, rather than a "dog-eat-dog" world.— jemima
- I believed the sun and the moon were the same thing, just rotated to change from day to night.— Daniel