Baking powder and baking soda. Both of them are used so frequently in quick baking projects that unless you are a recipe developer, rarely do you consider what each of them actually does for your finished product. How come my scones call for baking powder, but my buttermilk biscuits call for a mixture of powder and soda? Is there an easy way to substitute one for the other if I don't have both on hand? And why do I have to bake my muffins right after mixing the batter? This edition of the Food Lab is a quick and dirty guide to how they work, and how they affect the outcome of your recipe.