When I was a precocious 3 year old, I was certain that all adults spoke Martian among themselves.
I "proved" it to myself by slowly sneaking up on conversations between my mother & our neighbors.
Sure enough, as soon as they psychically detected my presence, the wa-wa underwater-sounds of Martian would smoothly segue into English.
A childhood belief about speaking

others believed things like this
- I used to think that adults spoke an entirely different language (not English) when kids weren't around. — Swordfish
- I used to think that even people who spoke something other than English, thought in English— Peaches
- I used to believe that everybody spoke English in the worldAnon
- I know my dog, Palmer, spoke English, he was just really quiet.
— Klayne P. - I used to believe that the whole world spoke the same language- English.Anon
- I used to believe that, no matter what language other people spoke, that everyone thought in EnglishAnon