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.
I’m not exactly sure how it worked in practice, but if it’s anything like simple aircraft engines with carbs, there’s a mixture control that you’d use in addition to the throttle to control air intake.