Baking powder is a leavening agent used in baking to help dough rise. It typically contains a combination of an acid, such as cream of tartar, and a base, like baking soda. When mixed with liquid and exposed to heat, it produces carbon dioxide gas, which creates bubbles and causes the dough or batter to expand. There are two main types of baking powder: single-acting and double-acting. Single-acting baking powder releases gas as soon as it is mixed with liquid, while double-acting baking powder releases gas in two stages—once when mixed and again when heated. This makes double-acting baking powder more versatile for various recipes.