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.
No one (!) alive today experienced a year divisible by 4 that was not a leap year. The oldest living person was born in 1907.
quite a few people alive today might be around to experience 2100, though