One-legged squats don't seem very tricky—after all, you did manage to pick up that cotton ball you dropped without putting your newly pedicured foot on the floor—but squatting on one leg seriously challenges your balance. It also activates your core and just about every other muscle in your lower body, including your glutes, hamstrings, and calves. Try it: Stand holding your arms straight out in front of your body and raise your right leg off the floor. Push your hips back and lower your body as far as you can. Pause, then push your body back to the starting position.