Every athlete knows it well: the soreness that sets in a day or two after a particularly tough workout. A recent article in the New York Times offers a somewhat helpful explanation of how it works (no one knows) and what you can do about it (not much). Known to sports scientists as delayed muscle soreness, the aches that arrive more than 24 hours after exercise may be caused by microscopic tears in muscles. Or, they may be caused by some other thing. As for pain relief, some athletes swear by Aleve or Advil, although research suggests that the help they offer is almost insignificant. One thing you should do, doctors say, is take pains to stay adequately hydrated during and after exercise. How will you know? Check the color of your urine. If your urine is dark, drink up. And keep drinking until its color returns to normal.