When exercising, the body muscle will produce heat causing an increase of the body temperature. When body temperature is too high, the homeostasis mechanism will be activated to reduce the temperature.
In humans, the mechanism is by sweating. Sweat will make the skin wet and the water from sweat will evaporate, taking the temperature of the skin making it cooler.
In dogs, the fur can insulate the skin so it won't effective. Dogs skin also don't have the sweat glands. The evaporation happens in their tongue. So, when the dog is feeling hot, they will take out their tongue so evaporation can happen.
