Baking powder is a chemical leavening agent that causes batters to rise when baked.  It consists of baking soda, plus an acidic salt, and cornstarch to absorb any moisture to prevent a reaction from taking place until a liquid is added to a batter.  Buy double-acting baking powder, avoiding those containing aluminum salts (for health reasons), store in a cool, dry place….and replace your supply every 6-12 months.  Double-acting means that baking powder begins its leavening action when liquid is added to dry ingredients.  The acidic salt (usually a phosphate) reacts with the baking soda as soon as liquid is added, producing carbon dioxide gas.  The second reaction takes place when the batter is placed in the heat of your oven.  The gas cells from the first reaction expand with heat to make the batter rise.  Once you add the liquid, you can only wait 15-20 minutes before baking without losing leavening power.  To test baking powder’s effectiveness:  mix 1 tsp with 1/2 cup hot water.  Mixture should bubble immediately.