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.
“You look unhealthy! You should go stand in that really large room and absorb the radiation from that gigantic space-based fusion reactor more!”
You’re right, that sounds like a great idea.