Woody: Refill, Mr. P.?
Norm: Yeah. [knuckles crack]
Woody: Eww! Knuckle cracking. I hate that sound. And you hurt your fingers.
Cliff: Woodrow, knuckle cracking is relatively harmless. Actually, when you crack your knuckles, what you're doing is really, uh, breaking a sound barrier. Yeah, what you hear is actually a tiny finger-sized sonic boom.
Frasier: Well, Cliff, you're partly right.
Cliff: Really? Which part?
Frasier: The phenomenon of knuckle cracking is relatively harmless. But in fact, the sound you're hearing is not a cracking at all, but rather a popping of tiny gas bubbles imploding in the synovial fluid of the metacarpal phalangeal joint.
Woody: Makes sense.
Norm: Yeah, but knuckle-cracking doesn't make a popping sound. A popping sound is more like this. [popping sound]
Woody: Oh, God, I could never do that. You know what else I could never do? I could never whistle with my fingers.
Norm: Carla can do that great. Hey, Carla, show Woody how you whistle with two fingers.
Carla: Sure. First, you have to crack your knuckles. [cracking sound]
Woody: Ooh, synovial fluid imploding! I hate that sound! You'll hurt your metacarpal phalangeal joint.