A friend of mine (at age 15, so I guess she should've known) used to believe that when you scream in a bottle and you close it, the moment you open the bottle, the bottle would scream. When we asked her how the hell she got to this thought, she simply said "Well I saw it in a cartoon, so I thought it could really happen". Well of course.
We still tease her about this one.
A childhood belief about cartoons

others believed things like this
- I thought that elmers glue was made from milk, since it had that cow on the glue bottle. — Monty
- that a piece of liver in your fridge if left overnight would wrap itself around a bottle of milkAnon
- My sibs and I thought that the screen door was called the "scream door" so Mom could yell at us through it. — Kyle
- i used to and still believe if you pee in the water scream will come and eat you— david
- "Yooce cream, ice bean, we all scream for ice cream!"
This song was always stupid to me.— Shirley B. - My friend's sister believed the lyrics to Gazza's "Fog on the Tyne" were "Fog on the dinasaurs mine all mine"— Suzanne