Many people would say that there’s no greater joy than curling up on a rainy day with a cup of tea, a good book, and a purring cat in your lap. Of course, some people have never experienced the unique happiness that comes with a contentedly purring cat, whether your cat is a non-purrer, or whether you may be allergic to cats—hopefully it is not because your cat is never happy! That being said, purring is not only caused by happiness, but can also be a reaction to a plethora of other factors in your cat’s life. Cats purr using their larynx and diaphragm muscles as they inhale and exhale, although how the central nervous system controls and sustains the vibration is still a mystery. Most cats do purr, from big cats on down to the household variety. There are thought to be only a few exceptions, and those are mainly from non-domesticated cats; it's not known if lions, leopards, jaguars, and tigers purr, although scientists are sure that cheetahs and cougars do. Most commonly, cats purr when they’re happy. This reaction starts between mother cat and kitten, as a way to establish a bond. While nursing, a kitten can’t meow, but a purr exchanged between the two is a way for the two of them to communicate and say “all is well.” Beyond this stage of growth, most purring happens when a cat feels secure and happy, but there are a few other circumstances when you can hear a cat purr. A surprising one occurs at the veterinarian’s office. Cats may start purring when in stressful situations, often to comfort themselves, and so when brought to the vet a cat may begin to purr for the examination. Because purring makes it difficult to hear how a cat’s lungs and heart are working, your vet may turn on the sink in order to hear better. This may seem counter-intuitive, but oddly enough, most cats will stop purring if they hear running water. Outside of the vet’s office, an older cat may purr when they approach other cats, especially strangers, as a way of signaling that their intentions are friendly. Cat owners have cited that their cats purr when hungry or when grooming themselves, but there may be even another reason why cats purr. The vibration may be used not only for communication, but also to promote tissue regeneration. In this scenario, purring is more unconscious than when being used to indicate happiness, ask for food, or as a comfort mechanism. Cats in the wild spend large portions of their days lying around, and so purring may be evolution’s solution to keep bones healthy. Purring is thought to stimulate a cat’s bones and keep them from becoming weak or brittle. This theory has begun to influence vibration therapy for humans suffering from particularly brittle bones. Researchers have even proposed attaching plates that would vibrate at the same frequency as a cat’s purr to astronauts’ feet during long space flights, in order to help their feet retain bone density. With all of these reasons for a cat to purr, the next time you’re cuddled up with a purring cat or kitten, try purring back! It’s one of the main ways your cat communicates nonverbally, and they may appreciate you signaling back that you also enjoy the time you spend together.