There are three types of baking powder: single-acting (also called fast-acting), slow-acting, and double-acting. Single-acting is made from an acid that will quickly dissolve and produce gases when placed in cold water. Slow-acting will not react until heated. Double-acting has both elements so that some leavening (about one-third) occurs during mixing and the remainder during baking. The latter allows a mixed dough or batter to be stored for several days with no loss of leavening power. Once opened, baking powder has a shelf life of up to six months. To test its viability, add 1 teaspoon to a cup of water. If it fizzes, it is still active.