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.
In the same vein, when I was a kid, all the death camp nightmare stuff about DPRK and the Uyghur genocide stories when it first came out
not facts
It's a commonly known "fact" that I don't believe because it's so bizarre like the parent comment about not believing the official 911 story