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.
But aren’t the tides caused by external gravitational forces (the moon?)
the tides stay in the same place relative to the moon and the earth spins below the tidal bulges (earth spins faster than the moon orbits, is the basic thing)