When I was about five years old, there was a commercial for Spaghetti-Os that ended with the announcer saying "with and without meatballs". I thought he was saying "withend without meatballs". I surmised that "withend" was another word for pasta or noodles, and at least once, I used the new word, much to the confusion of anyone who heard me.
A childhood belief about speaking

others believed things like this
- I used to believe that spaghetti grew on trees, based on a pasta commercial from the 70s.— Dianne
- I used to believe that meatballs were made from dead rats.— Dogface
- whenever the tv announcer said "brought to you", I thought it was one word - "brotue".Anon
- In first grade, I thought that if I've never heard of a word before, then it wasn't a word.— Destiny
- That baked beans were made of pasta! Spaghetti is?!Anon
- For years I heard the word "pedophile" and thought it meant a pedicurist's tool.— Lauren