When using baking powder, always check the expiration date. Expired baking powder will not help your recipe rise in the oven, and you'll have sad, deflated muffins. You can test if your baking powder has expired by adding ½ teaspoon baking powder to ¼ cup hot water. If the water bubbles up, it’s not expired; if it does not bubble, it’s expired and should be tossed.