Why do dogs kick when you hit the sweet spot?

The full narration of the video.

Scratch a dog's head, and it just squints, perfectly still. But slide your hand to one spot on the side of its belly, and a back leg starts kicking, faster the more you scratch. Same hand, same dog. So why does that one spot make the leg kick? A lot of people guess you hit the sweet spot, and it feels too good. Others guess it's annoyed, and trying to kick your hand away.

Both guesses assume the same thing: that the dog is the one deciding to kick. The clue goes back more than a century. In 1906, the British scientist Charles Sherrington studied exactly this leg. He mapped the patch of skin that sets the leg off. It's shaped like a saddle, covering the sides of the back, the ribs, and the flanks.

Scratch outside the saddle, like the head or the rump, and the leg mostly stays put. Stranger still: even when commands from the brain can't reach the back legs, scratching the saddle still makes the leg kick. In other words, the order to kick isn't coming from the brain at all.

So who gives it? A long cord of nerves running down the back, called the spinal cord. Nerves under the skin get scratched, and the signal runs into the spinal cord. Without waiting for the brain to weigh in, the cord tells the leg: Kick! The brain feels it too, but this kick doesn't need its approval.

You have this too: touch a hot pan, and your hand pulls back before it even hurts. When the doctor taps your knee with a little hammer, your leg kicks up on its own. So why would a dog come with a switch like this? The most common explanation: getting rid of bugs. Fleas and ticks crawl onto the sides of the belly, where the mouth can't reach. A few kicks can knock some of them off.

It makes sense, though it hasn't been fully proven. So that leg isn't really under its control. What you found isn't a sweet spot. It's a bug-kicking switch. So does it actually like it? Look elsewhere: if it rolls over and shows you its belly, it's probably enjoying it. If it wriggles away, it's had enough.

Vets even use the switch in reverse: they rub the base of a dog's ear, and if the back leg starts kicking, they check for mange mites. In one study, about eight in ten dogs with mange kicked, compared with fewer than one in ten dogs with other skin problems. Is there a spot on you that moves all by itself when touched?