Start on the floor with your knees off the floor, and your arms straight, facing down. Drive one knee towards your chest, then swap to the other leg. Try and keep your hips low and your back straight. As you get better at this, you will be able to go faster and faster. Stand tall with your legs apart and slightly bent. Jump both legs out to the side and then back, in and out (like the bottom half of a star jump, but keeping them slightly bent all the time). For a lower-impact version, tap one leg out to the side at a time.