A dog, like a cow, an elephant, or a man, is a mammal. All mammals are warm blooded, or homothermic animals. In the animal world only mammals and birds are warm-blooded creatures. The rest of the animals are cold-blooded or ectothermic. A warm-blooded animal has a relatively constant body temperature, while the temperature of cold-blooded animals changes according to surroundings.
Heat is being constantly produced in our body. A part of it escapes through our skin. We have sweat glands under our skin and sweat comes out through small pores in it. When sweat evaporates, it absorbs heat from the body producing a cooling effect.
Unfortunately, not all mammals can use this method to get rid of their body heat.
For example, a dog has few sweat glands and, therefore, cannot sweat like us. Similarly, tigers, lions, cheetahs, etc. cannot sweat much. Birds and rats also cannot sweat.
A dog has a different mechanism to control its body temperature. It does so by hanging its tongue out. When a dog pants, it takes in air through the nose and gives out air through the mouth. The saliva in its mouth evaporates. Evaporation of saliva produces a cooling effect and the dog feels comfortable. This habit of panting is simply a natural urge of a dog to control its body temperature in accordance with its surroundings.
An elephant also maintains its body temperature in a very interesting manner. Its skin is very thick and not much heat can escape through it. However, the skin on its ears is much thinner. The elephant, therefore, tries to get rid of the body heat through its ears. Nature has given the elephant very large ears which it can flap in a bid to lose heat. The skin of our ears being quite thin, the ears feel cold on a winter day.