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.
The thing that started this conversation is hemoglobin, which is also not metallic but a protein. I don’t think anyone was confused and thought that there’s actually shiney silvery elemental metallic calcium in our body.