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.
Time relativity always boggles my brain, I accept the fact but I find crazy that if I strap my twin and his atomic clock to a rocket and send them out to the stratosphere at the speed of light, when they return he’ll be younger than me and his clock will be running behind mine. Crazy