A dog that is sweating in hot weather will hang its tongue out to the side of the mouth to cool off. The dog’s sweat glands are on its tongue. Moisture that evaporates from the surface of the tongue cools the dog down. Sweating does not occur through the skin of the dog because of the layer of fur. If a dog were to sweat through the skin and fur, the dog would be almost constantly wet.
If the dog’s tongue hangs lopsided outside the mouth in cold weather, there may be some infection or inflammation inside the dog’s mouth. The dog might grimace.