Lie on your back and put your hands behind your head. Raise your legs so your thighs are perpendicular and your lower legs are just above parallel to the floor. Curl up and bring your left elbow toward your right side while drawing your right knee in to meet it. It is like you are riding a bike. Alternate sides, continuing the motion back and forth. Remember, don't just flap your elbow across your body, actually rotate your shoulder across and squeeze your abs.