Why does exercise make us happy and calm? Almost everyone agrees that it generally does, a conclusion supported by research. A survey by Norwegian researchers published this month, for instance, found that those who engaged in any exercise, even a small amount, reported improved mental health compared with Norwegians who, despite the tempting nearness of mountains and fjords, never got out and exercised. A separate study, presented last month at the annual meeting of the American College of Sports Medicine, showed that six weeks of bicycle riding or weight training eased symptoms in women who’d received a diagnosis of anxiety disorder. The weight training was especially effective at reducing feelings of irritability, perhaps (and this is my own interpretation) because the women felt capable now of pounding whomever or whatever was irritating them.