The soufflé is a light, airy baked dish that relies on the expansion of trapped air. It consists of two parts: a flavored base (like a crème pâtissière or savory sauce) and whipped egg whites. When the egg whites are whipped, the proteins uncoil and trap air bubbles. In the oven, the heat causes these air bubbles to expand, pushing the soufflé upward. The soufflé 'falls' when it is removed from the oven because the air inside cools and contracts. To help a soufflé rise straight up, chefs often use their thumb to create a 'channel' around the edge of the ramekin before baking.