Exercise is our modern obsession, and we have the fancy workout gear and fads from HIIT to spin classes to hot yoga to prove it. Exercise-a form of physical activity distinct from sports, play, or athletics-was an ancient obsession, too, but as a chapter in human history, it's been largely overlooked. In Sweat, Bill Hayes runs, jogs, swims, spins, walks, bikes, boxes, lifts, sweats, and downward-dogs his way through the origins of different forms...