You've seen dogs do it and you know it's called panting - even though dogs don't wear pants.

But have you ever wondered why they do it? You and I don't seem to do it. Cats and other

mammals do it very occasionally. But dogs and their relatives are best known for it,

and for good reason.

Panting is one way, the main way really, that dogs release heat in order to lower their

body temperature. Humans do this mostly by sweating, of course. When you get hot and

start to sweat, your body heat excites the water molecules on the skin. Some of them

get so hot that they evaporate away, leaving the cooler molecules behind. Those molecules

escaping into the air take the heat energy with them, leaving you cooler. So evaporation

removes heat from a liquid, which is how sweating cools us down.

Now you may have heard that dogs can't sweat, but that is not true. Dogs do have sweat glands,

but only their paw pads. And that is not enough surface area to keep the dog's whole body

cool, especially when they spend so much time standing and walking on their precious paws.

So to make up the difference, dogs release that heat through the evaporation of their

saliva, in addition to their sweat.

With all that hard and fast breathing, more water than usual evaporates from their mouths

and throats, and even their lungs. This removes a lot of heat really quickly, even from the

inside of their bodies. But it also means that they need a lot of water when they're

hot because they lose a ton of it through that evaporation.

And panting is more than just heavy breathing. Dogs also let their tongues hang out of their

mouths when they pant, not because they're tired or lazy, but because it makes their

saliva evaporate even faster. If you've ever gotten a loving lick from a pup, you've no

doubt noticed that their tongues are long and flat, creating a lot of surface area for

evaporation. They're also packed with blood vessels, so the blood can be cooled quickly

before reentering the core of the body.

So even though it's slobbery and messy, panting is a really remarkably complex and efficient

system. So why don't all of our furry friends use the same system then? Why don't we pant?

Well we do! All mammals, even humans, pant under certain conditions such as extreme heat,

over-exertion, or allergic reactions. But most of us have lots of other ways of keeping

cool. Cats and kangaroos ,for example, lick themselves and let their saliva evaporate

that way. Other animals, like pigs and hippos and elephants, coat their bodies with mud

and are cooled as the water evaporates from that.

So the next time your dog looks like this, give her some water, maybe a hug, and then

both of you take a moment to be thankful for the process of evaporation.

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