Exercise is good for our health, but that doesn’t make doing it any easier. Many of us dread it and put off working out. In fact, 50% of adults fail to meet the minimal weekly fitness recommendations. Harvard evolutionary biologist DANIEL LIEBERMAN argues in a new book that the reason so many of us are failing at staying physically fit is because human beings never evolved to exercise, so it hard to motivate ourselves to go for a run, walk, or to the gym. In Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding, Lieberman explores the science of physical fitness and the ways to make exercise more fun. He also debunks a number of myths about physical fitness, including that running is bad for your knees and sitting is the new smoking.