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.
This “choice” is just the manifestation what you are at that moment, the sum of everything that has influenced you up until this point. Whether that complex tangle of cause and effect was “determined” a million years ago or affected by random fluctuations the whole way, including a moment ago, doesn’t change anything. “Free will” just doesn’t make any sense, regardless of whether one considers predeterminism to be the alternative.