Have you ever baked a cake only to have it come out flat as a pancake? Before you use that baking powder that’s been in your pantry for who knows how long, check the expiration date and discard if it has expired. If you are in the middle of baking and want to use it, do this test to be sure it’s still active. Simply add 1-teaspoon baking powder and 1-tablespoon water to a small bowl. If it doesn’t immediately start bubbling it has expired. For double-acting baking powder you can check for the second reaction caused by heat by placing the same bowl in the microwave for 10 seconds. It should bubble up and become very thick and gooey. Be careful as it may bubble over the sides (as in picture). The best thing to do is check the expiration date on the can or bag every so often and if the use by date is near, buy more the next time you go shopping.