When a cat is content, relaxed, or happy, it makes a soft, rumbling sound called a purr. The vibration comes from deep in the cat's throat and sounds like a gentle motor running. If you've ever petted a cat and felt that buzzing sensation under your hand, you've felt a purr. Cats purr when they're being stroked, when they're settled in a warm spot, or sometimes even when they're eating their favorite food.

The word also describes any similar low, vibrating sound. A well-tuned car engine might purr smoothly as it idles. When someone says a machine is purring along, they mean it's running perfectly, without any rattling or grinding noises.

Interestingly, scientists still aren't completely certain why cats purr. While purring usually signals contentment, cats sometimes purr when they're nervous or injured, possibly to comfort themselves. Mother cats purr to their kittens, and kittens start purring when they're only a few days old. Whatever the reason, that gentle rumbling sound has made cats beloved companions for thousands of years.