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 one of those home experiments that is super easy to do and impress the kids. You just pour a bunch of cereal into a ziplock bag and move a magnet around on the outside until you start to see the little black dust appearing on the magnet.