As a bit of backstory, my Dad, as a child, skipped Kindergarten. Until around 46, my dad thought that the primary colors were just a myth, and you could mix something like green and orange to get red. We still tease him about this.
A childhood belief about physics

others believed things like this
- I used to believe that noodles that were dyed different colors such as green had been killed.— Tasha
- I thought "Orange is the New Black" was about wearing orange to funerals.Anon
- When I was little I thought you could get arrested if you accidentally skipped a number while counting.— Hailey
- i used to think belgium was a myth.Anon
- Do you really mean to tell me that father Christmas is a myth?
Finally......— Peter (Aged 70) - I used to think that "tease your hair" meant make fun of itAnon