When I was embarrassingly old--9 or so, I think--I used to believe that if you had half a glass of water and stuffed a washcloth into it, and the water rose to the top of the glass, you now had a full glass of water. My mother was unable to explain displacement in a way that satisfied me, so I held on to my belief persistently.
A childhood belief about physics

others believed things like this
- I used to believe that CD's were made of glass, until I learnt otherwise.Anon
- if you broke the glass on the tv, the cartoons could come out, or you could go in there with themAnon
- I thought that hydrogen was pure water (since I knew hydro- meant water) and H2O was impure some how— Hexetic
- I used to believe that if i splashed water on the wall during a bath, the water would turn into spiders— justin
- I used to believe that if I broke the glass on the tv screen, I could go into the tv show...— Karyne
- that bathroom tap water was recycled "potty" water from the toliet. — livingdeadgirl