For example, Why do onions make us cry? Well, when you cut an onion, sulfur-containing compounds within the onion combine and irritate your eyes causing them to tear. Nature did it like that so that predators wouldn't eat them. Why add coffee to beef gravy? Because the flavor compounds that form when you brown meat are the exact same flavor compounds that form when you roast coffee. And why shouldn't you marinate your steaks overnight? Because by the time the inside gets tender, the outside is mushy.