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.
Your bones are not dead rocks! They are living organs that happen to have a lot of sturdy calcium structures in them. They do a lot of other stuff besides hold your body up. They store minerals for all the other stuff your body needs minerals for; that’s why osteoporosis is even a possible failure mode. Your bone marrow produces white blood cells for your immune system, too.