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.
Probably one of the most memorable and pivotal moments in my life was when my college professor showed us the origins of relativity and how Einstein came to the conclusion that E = mc^2
It’s a proof that only took about 10 minutes to explain, and the mathematics really aren’t that difficult to understand by most people. The geniuses in the fact that Einstein started by explaining how calculating relative motion meant that time had to be a variable that could be different depending on who the observer was. This in itself is an incredible observation, but you can take this to the extent to literally prove that mass and energy are directly related to each other. It’s absolutely wild and one of the most sublime equations ever made.