To test the vitality of your baking powder, add a teaspoon of it to a third of a cup of hot water. If it foams and bubbles, it has enough oomph left. If it just sits there, it's time to run to the store or — here's a thought — you can make your own. Combine a tablespoon of baking soda with 2 tablespoons of cream of tartar and 1-1/2 tablespoons of cornstarch. It may not have the same potency as a good store-bought double-acting baking powder, but it will serve in a pinch.