When I was young I used to read books about shark attacks (some fictional some not). One day I read a story about a shark that swam up river into fresh water and attacked people. This story led me to believe that there were lake sharks and I would never swim in lakes because of it. (It didn't help me any that my best friend at the time found out and would try to scare me).
A childhood belief about general

others believed things like this
- I used to believe that once you pulled the plug in the bath a shark would swim out of the drain and eat you.— Amy
- I used to believe that the "shark mouth" nose art painted on military planes was to scare enemy pilots away.Anon
- I used to think that the TV show "Shark Tank" involved actual sharks in a tank.Anon
- I used to think that a "pool shark" was an actual shark that lived in a swimming poolAnon
- A friend believed that they used a tame shark during the filming of "Jaws".— Wendy
- I believed they put heated coils in lakes for the ducks to swim in the wintertime.Anon