Simple stomach- exercise

This is just a simple exercise that I faithfully perform everyday. Every morning when I wake up I lie down and raise my legs up and down so that it will form a 90 degrees angle. I do this for 20 counts. Counting is done when my legs assume its original position, so when my legs have completed upward and downward motion it is considered as one count. At night before going to bed I perform this exercise again. This is very effective because at 30 yrs. of age believe it or not my stomach muscles is still firm. Whenever I perform this exercise I still felt that my lower abdomen tightens because when I raise my legs I exerted the strength from my abdomen and not from my legs. You can try this and send me your comments