is nearly 800 times denser than air, each kick, push, and pull is like a mini resistance workout for your entire body -- especially your core, hips, arms, shoulders, and glutes. So in addition to blasting calories as you swim, you build lean muscle, which ignites your metabolism so that you burn more calories once you've showered and dried off. The irony is that while swimming makes you lean and mean, it's also kind to your body. Water basically neutralizes gravity, so you become virtually weightless when immersed, giving your joints a much-needed vacation. "You can swim almost every day without risking injury," says Joel Stager, Ph.D., director of the Counsilman Center for the Science of Swimming at Indiana University at Bloomington, who has studied the effects of swimming for years. "You can't say the same for running or strength training." And that makes swimming something you can do for your entire life -- a major bonus because it can literally help you stay younger: "Our research shows that habitual swimmers are biologically up to 20 years younger than their actual age," Stager says. The data, which were presented at an American College of Sports Medicine Conference, revealed that a swimmer's blood pressure, cholesterol levels, cardiovascular performance, central nervous system, and cognitive functioning are all comparable to someone far younger.