For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
It’s even weirder than that. “You” are a story that your brain tells itself so that you can explain your needs to other people. Without other people, or at least the pretend image of other people, there’s nothing like what we think of as a human personality.