You've cut your cancer risk. In a study of more than 14,800 women, those who had the highest levels of aerobic fitness were 55 percent less likely to die from breast cancer than those who were sedentary. Women considered moderately fit had about a 33 percent lower risk of developing the disease. Exercise may also help protect against endometrial, lung, and ovarian cancer, researchers say.

The skin also serves as a release point for heat. (See “Why Does My Face Turn Red When I Exercise?” for more on that.) When you exercise, your muscles generate a lot of heat, which you have to give up to the environment so your body temperature doesn’t get too high, Hackney says. The heat in the muscle transfers to the blood, which shuttles it to the skin; it can then escape into the atmosphere.