At the bottom of your stomach and top of your intestines, your food hits an important traffic signal: It’s the red light that tells your brain you’re full and don’t need another large order of onion rings (or the cheese sauce for dipping or the beer to wash it down). That red light is delivered by the vagus nerve, which is a large nerve that comes from the brain and stimulates the contraction of the stomach. The vagus nerve is also the main cable controlling the parasympathetic system, which is the relaxation section of your nervous system. YOU-reka! The key messenger switching the vagus on is a peptide produced in your gastrointestinal track called CCK, which is released when your bowel senses fat. Technically, it stands for cholecys-tokinin, but for our purposes, let’s think of it as the Crucial Craving Killer because its main purpose is to tell your brain via the vagus nerve that your stomach feels fuller than a Baywatch bathing suit.