Why do dogs pant with their tongues out?
Short answer
Dogs pant by breathing quickly with their mouths open. Evaporating water from the tongue and mouth helps them release body heat.
How it works
People can cool themselves by evaporating sweat from much of the skin, but sweating is not enough for dogs to cool their whole bodies. Panting moves air over moist surfaces in the mouth and tongue.
Panting can increase after exercise or in hot weather, and it can also happen when a dog is excited. Panting and drooling are among the ways dogs help regulate body temperature.
Key point
The important part is not simply sticking out the tongue; it is the evaporation of water from moist surfaces. The amount of panting varies with temperature, activity, and the individual dog.