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.
There are only 24 episodes of the initial run of The Jetsons and only 25 of Scooby Doo. They got aired as reruns for decades before more episodes were made. There are only 15 episodes of Mr. Bean.
This one startled me.