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.
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.
