Why do cats purr?

Your cat is humming. But why does it do that?

A purr is a soft, buzzing sound, a bit like rolling the letter r when you talk.

Inside the cat, tiny muscles in the throat twitch very fast, shaking the voice box and making the buzz.

The purr hums at about twenty five vibrations every second, so you can feel it on the cat's body.

• About 25 vibrations a second

Cats purr while breathing in and also while breathing out, so the sound never stops.

The most common reason is happiness. A relaxed, well fed cat purrs to say life is good.

Kittens purr too. A mother cat purrs to her babies, and they purr back to say they are safe.

Cats also purr to ask for something, like food, a cuddle, or the door to be opened.

But here is a surprise. Cats purr when they are scared, hurt or ill, to calm themselves down.

So a purr is like a hug from inside. It can help a cat manage pain and feel better. Purring is not only for pet cats. Wild cats purr too, and so do some other animals.

Other animals like raccoons and rabbits make sounds that seem like purring, but true purring belongs to cats and their cousins. Let's test what you learned. Cats purr to show happiness, to bond, and to calm themselves when they hurt or feel scared.

• True purr: cats and genets