Dogs do not cool themselves the same way humans do.
Humans have sweat glands spread across much of the skin.
Dogs rely heavily on panting.
Dogs do have sweat glands, but not enough for whole-body cooling
Dogs have some sweat glands in their paws.
Those glands can contribute a little to cooling.
But a dog cannot soak its entire coat with sweat the way a human can cover the skin with perspiration.
That would not work especially well under fur anyway.
Panting increases evaporation
When a dog pants, air moves quickly across moist surfaces in the mouth, tongue, and upper respiratory tract.
Water evaporates from those surfaces.
Evaporation requires energy.
Some of that energy comes from heat in the dog’s body.
The result is cooling.
Why does the tongue get so large?
During panting, the tongue can extend far out of the mouth.
That increases the moist surface area exposed to moving air.
More surface area gives evaporation more room to happen.
Blood flowing through the tongue and nearby tissues can then return slightly cooler.
Does panting always mean the dog is hot?
No.
Dogs also pant because of excitement, stress, pain, fear, exercise, medications, or medical problems.
The context matters.
A dog panting after running on a warm day is different from a dog panting heavily while resting in a cool room.
Why is humidity a problem?
Evaporation works best when the surrounding air can accept more water vapor.
Very humid air slows evaporation.
That makes panting less effective, just as high humidity makes human sweating less effective.
Heat plus humidity can therefore be especially dangerous.
Can dogs overheat even while panting?
Absolutely.
Panting has limits.
If a dog is producing heat faster than it can lose it, body temperature can rise dangerously.
Heavy panting, weakness, confusion, vomiting, collapse, or other signs of heat illness require immediate attention.
So the giant tongue hanging out is not the dog being dramatic.
It is part of the air-conditioning system.
A related animal question is Why do dogs tilt their heads?, which shows another way biology shapes familiar behavior or anatomy.
And that’s the short explanation.
Related explanations: Why are dogs’ noses usually wet?, Why do dogs shake after getting wet?, and Why do dogs tilt their heads?