what makes knuckles crack is carbon dioxide bubbles in your knuckles along with cartilage.

The sound you hear when you crack your knuckles is actually the sound of carbon dioxide gas bubbles bursting.

http://en.wikipedia.org/wiki/Knuckle_cracking

No he doesn't crack his knuckles. That's just his character name.

You can't. They get bigger as you crack them. Don't crack your knuckles or you'll resort to big knuckles!

Not everyone can crack their knuckles. About half of those people who responded to an online poll could 'always' crack their knuckles, while a third could 'sometimes'. That leaves about 15 percent that said they could not crack their knuckles.