Water is about 12 times as dense as air, so each stroke and kick through it is a mini strength-training exercise. When you swim, every muscle in your body is called into action as you push against the water's resistance. What's more, regular swimming can make you more flexible, something active women struggle with as they get older. Since you weigh only 10 to 15 percent of your land weight in the water, your arms and legs feel lighter and your range of motion increases as you move. Swimming also lengthens your body to its fullest potential, strengthening and stretching otherwise tight, shortened muscles.

The old myth that swimming doesn't burn off fat because your heart doesn't work as hard and your body hangs on to your excess "insulation" in the cool water is just that--a myth. True, your working heart rate can be reduced by as many as 17 beats--or by 10 to 15 percent of your normal working rate on land. But that's because the force of the water actually assists your circulation in helping your heart do its job, and your body temperature is lower in the water, which automatically lowers your heart rate.

But that doesn't mean you're not getting a workout. Your lungs actually work harder against the pressure of the water, and your muscles put in overtime keeping you afloat. As a result, you're burning as many calories and conditioning your heart just as well as you would if you were doing most dry land exercises. Your lungs are getting an even better workout.

Swimming is the best way to boost a sagging butt. You see those tight buns in the tank suits of pro swimmers because every kick is powered by the glues; every lap firms and lifts those muscles. Keep up this butt-building and fat-burning exercise, and you just might find yourself breaking out that thong...at least for private viewings!