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.
It makes a lot more sense if you stop believing in the fiction of objects. There are no objects. Particles are a fiction, waves are a fiction. There is a single process, that we call the universe, and every single thing we refer to is a portion of that process from a subjective perspective. Once you give up on objects the idea that a process can be observed to produce subjective experiences that appear to violate expectations of object-oriented conceptual frameworks becomes less difficult to grasp.