After I learned what H20 meant, I thought that faucets worked by combining hydrogen and oxygen atoms in the pipe when you turned it on.
A childhood belief about chemistry

others believed things like this
- I thought that hydrogen was pure water (since I knew hydro- meant water) and H2O was impure some how— Hexetic
- I thought if someone worked full time it meant they worked 24/7 and never did anything else!— andalite
- I thought the white circles you see when looking at a bright window were what atoms of air look like.— H.
- I used to think that there were small people on TV who worked when I turned on the TV— JAVIER
- i used to believe that my brain worked like the inside of a clock. How the wheels cranked and turned.— Becky
- I used to think that oxymoron had something to do with oxygen...Anon