Crunches build the muscle. But you won't have a visible six pack is with just crunches. You have to do some conditioning training to lose the fat. Run, jump rope, wrestle..., just about anything that gets you heartrate up over 140, I'm not sure what the actual number is but it's around that, to burn calories and fat. Then when you lose the fat and do have a visible six pack, you have to have proper nutrition to keep it.