This is a great way to work your complete midsection (abdominals and obliques) and the only equipment you need is an exercise mat.
Lay flat with your fingertips resting lightly behind your ears. Move opposite elbows to opposite knees and crunch, squeezing your muscles as tight as you can then rotating to the other side. Continue back and forth. Your legs will be moving as if you are riding a bike and you will feel a good burn in your tummy.