For a long time, researchers have known that physical activity increases (1) ______ of life and prevents depression. But the (2) trouble 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) interested.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)long run, doctors can be more confident when they tell their (7)cheerful patients to start exercising, rather than giving them pills.