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.
The crazy thing is there are actually two double slit experiments, and that light can tell whether or not you are actively observing it or not, and decides whether or not to actually exist as a wave or particle.
I heard from someone I respect irl that these experiments were debatable, but I can’t personally hold an argument about it.