For a long time, researchers have known that physical activity increases (1) ______ of life and prevents depression. But the (2) ________ was that they didn’t know exactly why. Now, however, we have the answer: exercise increases levels of phenylethylamine. This is a chemical in your body that puts you in a good mood and makes you feel more (3) ________ .Whether you exercise is something you do (4) _______, like perhaps jogging, or a more (5) ______-team sport, like rugby, phenylethylamine will make you happy. So in the (6) _______, doctors can be more confident when they tell their (7)________ patients to start exercising, rather than giving them pills.