When I was little, like 3 years old I guess, I thought dogs were male and cats were female. I thought horses were female and cows were male. I thought the hood ornament on a car was to help the driver aim, and wondered why it was in the middle, not in front of the driver.
In kindergarten, I met a kid who thought the blue in the sky was the clouds, and the white was clear sky.
In third grade, my best friend told me, seriously, that he invented the word diarrhea. Actually, I can’t remember if it was the word or if he claimed to invent diarrhea itself.
I may come back and add to this list later.