Animal specialists believe that cats purr to show contentment. Cats are born with the ability to purr; kittens make tiny rumbling sounds when they are nursing. Scientists think that purring starts out as a form of communication between a mother cat and her kittens. The purr lets the mother cat know that her babies are happy and feeding well, and she may purr back in response. Later, cats continue to purr when they are in a contented mood or as a friendly greeting.