What's something you believed to be true but recently learned is actually false?

I used to believe that common sense existed. You know, the usual stuff, like water is hot and fire is wet…

But then it occurred to me a few years ago, that what people believe to be ‘common sense’ are actually the things that nobody bothers to teach the next generation.

Meaning that common sense is only as common as one’s elders teach you. So when the elders assume that you automatically know certain things, they won’t bother teaching you.

Hence, common sense does not exist.