Your beautiful, beautiful hands.
Almost any movable joint can be "cracked" deliberately, from fingers, to toes. Now here's how it works is , when you crack your knuckles, you put force onto your joints which pull apart the joints between the bones, (Now I did't say dislocate, or split, I said pull.) When your joints are pulled apart, the connective tissue capsule between your joints are stretched. When the tissue capsule is stretched, you increase the volume, and as most people know who has ever taken chemistry, or has basic knowledge of such would know, when you increase volume, you decrease pressure.
More beautiful hands, plus science!
Inside of the tissue capsules are fluid that contain gases, when the pressure drops, the solubility of the fluid is decreased, and some of the gas forms bubbles. If enough pressure is applied, then the bubbles burst creating the cracking noise. It takes 25-30 minutes for the gas to redissolve and former volume and pressure to return to normal. After this time, you would be able to crack your knuckles, although during the time period, you would be unable.
Now we've all heard about cracking your knuckles gives you arthritis, and nobody wants that! Well, good for you, because it does not give you arthritis. In fact, right after cracking your knuckles, you get increased mobility, its just like what the chiropractor does to ones back/neck.
Though it isn't all flowers and ponies, cracking your knuckles can generally weaken 25% of your grip strength, and hand swelling. Also, outside of physical health, its pretty annoying for that kid in class that cracks every part of his body before a test isn't it? As we all know, the more often we crack our knuckles, the easier it becomes, for some people, they can just extend their leg, and it would "crack", imagine being unable to not crack your knuckles by simply squeezing your hand ,or extending your hand to shake someones hand, its pretty creepy.