Okay, first of all. Nobody can know something if they just don't know it. There really is no such thing as a stupid question. Only condescending answers. People ask questions because they don't know the answer, especially kids!
Second of all, calling a seven year old an idiot?? That's low class.
Oh didums.
Well, I'm sure he has alot of time to develop his other faculties, but hopefully this kind of thing will be more self-evident to 7 year olds in the future.