Any cat owner (cat owned?) has heard/seen this: the chittering toothy noise cats make when they stare out the window at a bird.
Why do cats chatter their teeth? Answer: they want to kill.
Seriously. They want to kill that bird they're staring at. The chattering noise & movement is an imitation of the cat's kill bite. When a cat hunts, after she catches her prey and has it securely in her paws, she bites down on the critter's neck. The rapid jaw movement is how the cat severs the prey animal's spinal cord, killing it instantly. This allows the cat to eat its meal without risking injury. It's also nicer than munching on the critter while it's still alive.
That's right. That cute teeth chattering noise we all love is our cat imitating his/her kill bite. Isn't that sweet?