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.
Downvoting because of “by the government”. Your biological sex is assigned by luck, depending on what chromosomes you get. Your gender on your birth certificate defaults to the way the “majority” of people are. But you’re not the majority. No single person is the majority. In fact, the only trait we are certainly sharing with a majority of people is the fact that we’re all unique (and the fact we’re all human).