A general theory of cookies may be formulated this way. Despite their 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 form better. In the cookie the agent of cohesion has become some variation of the theme of oil. Oils, be they in the form of butter, egg yolks, 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.

Oils in baked cakes do not behave as water in the finished product. Rather than evaporating and thickening the mixture, they remain, saturating the bubbles of escaped gasses from what little water there might have been in the eggs, if added, and the carbon dioxide released by heating the baking powder. This saturation produces the most texturally attractive feature of the cookie, and indeed all fried foods: crispness saturated with a moisture (namely oil) that does not sink into it