Exercise, as you no doubt know, is a late arrival in the history of civilization. Until around 1910, people exercised all the time, but they didn't think of it as exercise—they thought of it as life itself. They had to get from one place to the other, usually on foot, and harvest the crop, and wage war, and so on. But then the automobile was invented (not to mention the Sherman tank), and that, plus the Industrial Revolution, pretty much led to what we have today—a country full of underexercised (and often overweight) people—and a parallel universe of overexercised (but not necessarily underweight) people. I myself swing between the two universes. I spend time getting into shape, and then something breaks, and then I spend time recovering and not being in shape, and then I recover and I get into shape, and then something new breaks.

So far, in the breakage department, I have managed the following: I pulled my lower back doing sit-ups, I threw out my right hip on the treadmill, I got shin splints from jogging, and I entirely destroyed my neck just from rolling over in bed. A few years ago, during a wild and committed period of exercise, I happened to be sent a tape of the movie Chicago, and I made the mistake of confusing it with an exercise video. It was, without question, the greatest exercise video I have ever worked out to. I could lift weights forever while watching it. For the first time in my exercising life, I was never bored. I could be Catherine Zeta-Jones, and then I could be Renée Zellweger. I pranced around the apartment waving my five-pound weights here and there and singing "All That Jazz." I have never been happier exercising. But after three weeks, I woke up one morning in horrible pain and discovered I couldn't move my arms. Millions of dollars in doctor's fees later, it turned out that I had not one but two frozen shoulders, the result (naturally) of lifting too many weights for far too long. It took two years for these frozen shoulders to mostly thaw, and in the meantime, I had pretty much resigned myself to the prospect of never being able to scratch my own back (or zip up a dress) ever again. (Not that I wear dresses, but if I did.) But I have taken up exercising again. I have a trainer. I have my treadmill. I have my TV set over the treadmill. I exercise almost four hours a week and I would rather be in Philadelphia (although not in labor).

Next: Skin