U-Boat

This may be a small movement, but it works your lower abdominals in a big way!
How to do it: Sit with your knees bent, feet flat on the ground. Lean back to prop your upper body up on your elbows (your back should remain lifted), palms facing down. Brace your abs in tight and lift your legs into a 90-degree angle (knees should be touching), toes pointed. Slowly bring your legs over to the left (both hips should remain on the ground). Keeping the 90-degree angle with your legs, lower your legs and then lift them up to the right, as if you were tracing a letter 'U' with your knees. That's one rep. Do 20 reps total, alternating sides each time.
Make it harder: The longer the lever, the harder this gets, so if you want more of a challenge, try extending your legs out further (even straightening them completely).
Mind your muscle tip: Use your breathing to help deepen the abs work: inhale as you sweep your legs over to one side and down, and then as you come up to the other side, exhale and visualize your belly button pressing all the way back to your spine.source by www.shape.com