What truisms are so ingrained it seems like the natural order of things?

In the US and maybe other Western nations, we “know” that pink is for girls and blue is for boys, yet only a few centuries ago it was the other way around. But now, if we see pink on a boy, it makes a lot of people very uncomfortable.

In some Middle Eastern and Arabic or African cultures, if you eat using your left hand, it is considered very, very disgusting. A great faux pas. For that’s the hand you wipe yourself with, and there is precious little water.

I’m looking for examples of things (other than religious beliefs) that a large number of people accept as “truth;” where belief in it is so strong, it can not be imagined any other way. If it were another way, it would be unnatural. I’m looking for things that arise from a culture, yet feel like a law of nature; customs that hardly anyone ever questions.