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.
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Also the idea that light is both a particle and a wave always messed with my head because I wanted to know why does it decide to change and when? And the answer is that light is always a particle and always a wave at the exact same time.
It is a wave particle.
And it is possible from light alone to build both an electron and a positron as demonstrated in a 1999 laser science experiment in New York.
I usually interpret this as behaviour: photons are not “particles” or “waves”, photons are photons. They just behave as waves and as particles, depending on how you’re looking at them.
Note that even things with a resting mass (like you or me) are like this, too. It’s just that, as the mass increases, the wave behaviour becomes negligible.
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This doesn’t make it any less crazy
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.