After a day of skiing, bicycling, or swimming, some people find they get headaches — something like a hangover from drinking too much the night before. Even though exercise is healthy and excessive drinking is not, the headache in either case is your body’s response to the same problem — dehydration. First, another small mystery. If you commute by bicycle, you may find that you don’t seem to sweat very much until after you arrive. Since sweat is your body’s cooling system, you may not sweat very much until your body has begun to heat up. When you get to work, your body is still trying to cool down and so you keep on sweating. But even then, you aren’t really sweating any more than you were toward the end of the ride. It only feels that way because the wind that evaporated the sweat when you were riding is now gone. Swimmers sweat too, but, because they’re in the water, it doesn’t feel like it. And since high altitude increases the rate of evaporation, downhill skiers also may sweat far more than they realize.