Whenever a certain part of the body is in distress, it conveys its needs to us by sending signals to the brain, as when the gnawing sensation in our stomach warns us that we are hungry. In a similar way the tissues, whenever they lack water, can signal their need to the tongue and the throat which themselves begin to feel parched and dry. The tongue and throat are tissues through which we become aware of the lack of water most keenly. From this we not only know that our mouth is dry but that our whole body is in need of water.
Nothing else seems to quench thirst so well as water does.