Want to burn fat quickly? Rapid-fire circuits turn strength moves into calorie-torching cardio work. "If your goal is weight loss, use light weights and high reps," says exercise physiologist Pete McCall, of the American Council on Exercise. A circuit may include pushups, pullups, and crunches, followed by a 2-minute run. Repeat or alternate with another circuit of biceps curls, dips, and shoulder presses to target smaller muscles.