Powdered sugar, also known as confectioners' sugar, is a type of sugar consisting of very fine particles. Some types of sugar with small particles, such as castor sugar, are granulated sugars that are naturally very finely granulated. Powdered sugar, on the other hand, is regular white granulated sugar that has been crushed into a very fine, almost dust-like powder. It also contains a small amount of cornstarch to prevent it from clumping and hardening. If this cornstarch fails in its intended task, however, you may need to soften your powdered sugar before you can use it.
Put the powdered sugar into a large mixing bowl. Use a large spoon to break it into smaller pieces and soften it as much as possible.
Place the pieces and any powder from the bowl into a flour sifter. Hold the sifter over a container in which you want to catch the sugar.
Turn the crank on the flour sifter. This should break up the small clumps of powdered sugar, causing perfect powdered sugar to fall out of the bottom of the sifter. Continue turning the crank until all of the sugar has returned to its original powdery state.