A combination of ingredients, including baking soda, an acid such as cream of tartar, and a moisture absorber like cornstarch. An important chemical process occurs when baking powder is added to wet ingredients; carbon dioxide gas bubbles are released, which in turn cause the bread, cookies, or cake to rise and lighten in texture. The double-acting baking powder used by home cooks acts twice: some gas is released when it comes in contact with the wet ingredients and then again when exposed to heat in the oven. Baking powder is fragile and loses effectiveness quickly. A good rule is to replace it at least once a year to guarantee baking results.