Lying on your back, bring your legs into a butterfly position—knees out with the soles of your feet touching. Relax your legs. Bring your hands, fingers laced, behind your head, leaving your thumbs on your neck as sensors to keep your neck relaxed. Using your abs only, lift your upper body up two inches and back down 25 times. Then hold your upper body up and lift your legs two inches from the ground, tapping the sides of your feet back on the ground 25 times. For an advanced variation, raise and lower upper body and legs simultaneously.