Universal Press Syndicate No one is really sure exactly how cats purr.

A vibration, sure. But where? The rattling of skin folds, say some, while others argue that the sound is the movement of air through swollen blood vessels. And why is our cat the only one of his family to manage this lovely sound? Tigers, for example, can rumble and roar, but only the domestic cat can keep the motor running on both inhale and exhale.

When I think of purring, though, I don't think of science. Instead, I remember a day when a little cat saved his own life with the power of his purr.

A friend and I had gone to the shelter to search for an elderly neighbor's lost calico and found ourselves sadly pondering a cage bank full of homeless cats.

And then we heard the purr. It came from a leggy young tabby with a battered look about him. He revved up his motor as we approached, and in seconds we were both smitten with the half-grown kitten. Once out of the cage, he was as charming as his purr suggested, bumping my friend's chin with his head and settling into her arms as if he belonged there. Just that quickly, the purring tabby earned himself a new home.

The neighbor's missing calico wandered home the next day, thinner than when she left but otherwise unharmed. And the ratty little tabby with the big purr grew into a handsome and loving companion, which he likely would never have had the chance to become were it not for his purr.

The purr is the essence of all that is wonderful about cats. And although I am no scientist, I have my own theory of where such a beautiful sound originates. I have no doubt that it comes straight from the heart.