A general theory of cookies may be formulated this way. Despite its descent from cakes and other sweetened breads, the cookie in almost all its forms has abandoned water as a medium for cohesion. Water in cakes serves to make the base (in the case of cakes called "batter as thin as possible, which allows the bubbles – responsible for a cake's fluffiness – to better form. In the cookie, the agent of cohesion has become some form of oil. Oils, whether they be in the form of butter, vegetable oils, or lard, are much more viscous than water and evaporate freely at a much higher temperature than water. Thus a cake made with butter or eggs instead of water is far denser after removal from the oven.

source:[http://indiafoodnetwork.in/food-stories/national-cookie-day-5-of-our-favourite-cookies-in-mumbai/]