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 fact that calcium is a metal is the reason why bones can be detected in X-rays.
(I'm pulling this out of my ass and I'm too lazy to look it up to see if it's actually true.)
It's true!
Source: my ass too!