Since I was in the neighborhood on an errand, I decided to visit the studio where my husband Steve takes his spinning class. As soon as I walked in and saw a dozen people pedaling like crazy on stationary bikes, I was struck by one thing: sweat. Buckets of it. There wasn't a dry patch of skin in the room. No wonder someone was always reaching for a water bottle.

Gone are the days when conventional wisdom held that drinking while exercising is bad for you. Thank goodness, most people now understand that they must replenish fluids lost through perspiration. And according to polls, they also seem to accept that they should consume about two liters of hydrating fluids a day. So why do only about half of all Americans actually drink that much? Maybe they don't just realize how much good water does in our bodies, or the potential dangers of not drinking enough.

A component of every cell, water makes up 55 to 60 percent of our total body weight. We can survive three months without food but only a few days without water, which carries nutrients from the digestive system to all cells in the body, as well as cellular waste products to the kidneys so that they can be excreted in urine. Water serves as the solution in which all other nutrients are dissolved and as a "shock absorber" inside the eyes and spinal cord. It lubricates and cushions around joints and, through perspiration, regulates our body's heat.