Inspired by another current thread: what things have you thought to yourself, and accepted as true, which on telling someone else have quickly transpired to be completely ridiculous?
E.g: I always thought that when drinking a hot beverage, that the misty effect observed should you happen to glance down into the cup was your EYEBALLS getting STEAMED UP in the manner of a pair of glasses. When I casually mentioned this at work everyone kindly pointed out that I was just... seeing the steam.
My car is an old banger with no air con, just the air blower. For my entire life until my ExDP corrected me, I thought you had to 'run' the hot air until it turned from cold to hot eventually in the same way you do the tap. Rather than just turn it on once the car's warmed up. The hours I must have spent grimly tolerating a stream of freezing air in winter 
.
I'm not normally a simpleton by the way, I've got degrees and stuff and mostly manage to function.
So please tell me I'm not alone!