I sat down with Dr. Jordan Metzl, a sports-medicine physician who's convinced that the reason most of us can't stay consistent isn't discipline or knowledge, it's motivation, and he says you can actually engineer it by "lowering the cost to act." We get into exercise as a literal medicine that would beat almost any drug if it came in a pill, why fun is the real holy grail of compliance, and how movement can go head-to-head with antidepressants for everyday anxiety and depression. If you've been white-knuckling your way through willpower and losing, this one's going to change how you think about getting off the couch.