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.
We also distinguish between metals and non-metals by field of study. Ask an astronomer which elements are metals sometime.
How so? I thought they were mostly determined by their positions in the table of periodic elements.