Like a few others here, I used to think as a small child, that there was a monster under the bed that would drag me under if I let my legs dangle, and also one down the plughole in the bath, that would suck me down when the last bit of water drained out. So I used to shoot out of the bath before it emptied, right up to when I was about 8!
ALSO used to think all dogs were boys and all cats were girls.
Thought the pee came out of my actual vagina, (not the completely separate peehole.) I thought that til I was about 17! (Didn't pay enough attention in Biology!)
I couldn't understand why the steering wheel wasn't in the middle of the car, but actually on the right.
I was scared if I ate seeds/pips from apples or grapes that apples and grapes would start growing inside my stomach!
Also, I used to think that when people had sex, the man would put his willy in the woman, and the baby came out when his willy came back out. 😬I thought that from about 9 when I first heard about sex, (from older cousins,) to about 13!