Bodyweight Squat

Stand with your feet a shoulder-width apart (or slightly wider). Ideally, you want your toes pointing straight ahead, but if your hips are not very flexible, you can turn your toes slightly outward. In one motion, make as if you are sitting down, as deeply as you can, and then stand straight up. Do not let your heels lift up off the ground; instead, press up through the heel to come to standing.

Go as low as low as you possibly can. The deeper the squat and the lower your hips are below your knee, the less pressure on your knees.

This is a foundational move. Working the larger muscle groups burns more calories.