Surely this is something you can think about? Of course. We put a Cup or spoon with liquid to his mouth and “drawn” in their content. Here is something simple “retraction” of the liquid, to which we are accustomed, and the need to explain. Why, in fact, the liquid rushes to us in the mouth? That it carries? The reason is this: when drinking, we are expanding the chest and razrezaem the air in the mouth; under the pressure of the external air , the liquid flows into the space where the pressure is less, and thus enters our mouth. Here is the same thing that would happen to the liquid in communicating vessels, if one of these vessels, we have to pad the air pressure of the atmosphere, the liquid in this vessel would rise. On the contrary, having grasped the lip of the bottle neck, you no efforts do not “draw” the water out of her mouth, as the air pressure in the mouth and over the water equally.
So, strictly speaking, we drink not only by mouth, but also easy; after all, the expansion of the lungs - the reason that the liquid flows into our mouth.