When I learned that a piece of glass that you look through was called a window, I naturally assumed that a piece of glass you see your reflection in was called a mirrow. It never made sense to me that the words for such similar things had different endings. Still doesn't.
A childhood belief about speaking

others believed things like this
- i used to believe that one piece of ear wax was actually called a wach. — rachael
- The song called heart of glass mixed me up as a kid. I heard "Heart and ass". — oops
- I used to believe that CD's were made of glass, until I learnt otherwise.Anon
- My cousin once told me that glass was made from ground up beetles, and I believed her! — benjymous
- Used to wonder why a hamburger is called that if it doesn't have any ham...— Axecutioner
- if you broke the glass on the tv, the cartoons could come out, or you could go in there with themAnon