it will feel to be twenty pounds lighter or to play “Smells Like Teen Spirit.” Then after a while, reality hits. It’s hard to find the time to work out or practice as much as you should, so you start missing sessions. You’re not improving as fast as you thought you would. It stops being fun, and your resolve to reach your goal weakens. Eventually you stop altogether, and you don’t start up again. Call it “the New Year’s resolution effect”—