Purring is one of the most basic things cats do.
However, scientists are still not sure how this works. Most veterinarians believe it has something to do with a severe vibration of the vocal cords. A muscle in the larynx opens and blocks the passage of air at a rate of 25 times per second.
I have a black cat, his name is Spot, he's not happy unless my arm is bleeding from several puncture wounds for "loving me so much" in the morning.