Curling up with your favorite ball of fur as she purrs away is pretty close to heaven, at least for cat folk. Yet, hidden between those vibrations, that most appealing of domestic sounds remains wrapped in mystery, and even a little magic.

No one is certain exactly why cats purr, though there are a number of good guesses. The obvious observation is cats seem to purr when they're pleased and feeling good. But that's not always the case: Some cats also purr when they're hungry, injured, or frightened. And most surprisingly, purring frequencies have been shown to stimulate bone regeneration—yes, bone regeneration.

Cats purr by using their larynx and diaphragm muscles, both as they inhale and as they exhale, although just how the central nervous system generates and controls those contractions isn't yet understood. Early 19th century taxonomists thought cats could either purr or roar, and split the family Felidae along these lines—"purrers' (subfamily Felinae) and 'roarers' (subfamily Pantherinae).

A cheetah. Getty Images

Today, though, taxonomists believe most cats can purr, with a few probable (though not certain) pantherine exceptions: lion, leopard, jaguar, tiger, snow leopard and clouded leopard. (Cheetahs and cougars? Yeah, they purr.)

So, why do it? If it's a form of communication, it's meant for those near and dear, since cats purr at a frequency and volume too low to travel far. Purring (and many other low-frequency vocalizations in mammals) often are associated with positive social situations: nursing, grooming, relaxing, being friendly.

More likely, though, purring is simply soothing, or self-soothing, as cats may also purr in stressful situations. In that case, purring would be akin to how humans soothe themselves by crying, laughing, distracting themselves, or even organizing their desk. Some veterinarians and cat enthusiasts have observed cats lying alongside each other and purring when one is injured (a behavior termed "purr therapy"), though scientific literature on the subject is scant.

Beyond being calming for the injured kitty, "purr therapy" may have bone healing properties. Domestic cats purr at a frequency of about 26 Hertz, in a range that promotes tissue regeneration. That's not as crazy as it sounds: High-impact exercise promotes bone health for the same reason, because bones respond to pressure by making themselves stronger.

In their natural setting, cats spend a lot of time lying around waiting to hunt, so purring may stimulate bones so that they don't become weak or brittle. In fact, purr-like vibration devices have been patented for potential use in therapy, and some researchers have proposed strapping vibrating plates to astronauts' feet during long space flights to retain bone density.

Such reasons to purr are by no means exclusive. "All behavior depends on history, context and expectation," says Tony Buffington, a cat expert and veterinarian at Ohio State University. "So it's naive to think that cats can only purr for one reason—it's like thinking that people can only laugh for one reason."

Humans can laugh out of joy, a desire to be polite, when surprised, in discomfort or in derision—and only context will tell an onlooker which is going on.

It would be easier to tell what function the low-frequency rumbles play if we could "de-purr" a cat. But, Buffington notes, what are you going to do, cut off its air supply? You'd lose a cat and learn nothing. If you want to know why your particular favorite feline is purring, Buffington suggests paying attention to what prompted the purring and what it leads to.

Did you just arrive home to be greeted by your cat purring and rubbing against your leg? He might be happy to see you. Is your cat pestering you around dinnertime, purring insistently? It may be hungry. (Cats seem to purr with greater urgency when hungry.) Is he purring at you through YouTube? Maybe he's encouraging you to finish the story you're writing. (This was my soundtrack for writing this piece. You are welcome.)