Regular exercise helps the living organism remain accustomed to change, keeping the organism's ability to adapt to change functioning smoothly. After all, the essence of exercise is change: You change from a sedentary to an active state whenever you get up out of the easy chair.

During a walk, for example, or a jog, a tennis game, or an aerobics class, your body and mind are constantly being called on to change and adapt to continually altering conditions, some of them internal, some external. Weather, terrain, a tricky opponent -- all of these call for adaptation.

Over time -- a period of weeks, months, or years -- your body is called on to make further adaptations as you become fitter, as you increase your pace, and duration and intensity of effort. Altogether, these changes provide us with practice in adaptation, helping to keep our adaptive skills honed to a fine and effective edge.