Gina Price, age 10, of St. Augustine, Fla., for her question:

WHY DO WE USE BAKING POWDER?

Baking power is a chemical mixture. When you add baking powder to a cake or muffin batter, it starts to dissolve in the liquid. Then the chemistry begins. Baking powder makes baked goods puff up or rise so they are light, fluffy and tender when baked.

Bakers also use yeast in making bread light and fluffy. But yeast takes much longer to do its work of leavening, or making the batter light. Baking powder acts quickly.

There are two powders in baking powder: baking soda (or sodium bicarbonate) and a weak acid. The acid reacts with the alkaline soda. This produces hundreds of tiny bubbles of gas, like those in soda water. The gas is carbon dioxide.

These bubbles rise through the mixture and expand when heated, so they lift up the batter. By the time the food is baked, bubbles have formed a network of little holes all through it. This makes the food light and spongy.

There are three kinds of baking powder: phosphate, tartrate and combination. They are named for the acid they contain.

All baking powders are wholesome. They all give about the same amount of leavening gas. But some act quickly while some act more slowly.