Flutter Kicks

Lying down, face up, lace your fingers behind your head, or you can place them under your hips. Lifting your feet off of the ground, perform a slight "kicking" motion with your legs, almost as if you were in a swimming pool. Keep your legs straight and do not arch your back.

It's tempting to use your back in this exercise. Make sure your back is flat on the ground and your abs are continually engaged.