Roll over onto all fours, wrists under shoulders, arms straight. Extend one leg behind you, then the other, so your ankles and legs are together and straight. Pull your abs in, lowering your hips so your body forms a straight line from head to heel. Inhale and lift your left leg to hip height. Exhale, contracting your abs as you draw your left knee diagonally toward your right elbow; let your spine curl slightly as if you were doing a crunch. Inhale and straighten your leg to lower to the starting position. Repeat, alternating sides. Do 5 reps on each side.