Have you gone to bake something only to find your brown sugar is hard as a rock? To soften it is quite easy. All you have to do is place the block of brown sugar in a microwave safe bowl. Cover it with a slightly damp paper towel and microwave it in 20 second intervals, using a fork to break it up in between. Continue until the brown sugar has returned to it normal consistency.
The reason brown sugar hardens is because of the exposure to air. When this happens, the molasses loses moisture and dries out which in turn causes the sugar crystals to stick together.
To prevent this from happening in the future, simply store brown sugar in an air-tight container in your refrigerator the minute you open the package. You can also do this in your freezer to preserve the freshness, but you'll have to remember to let it come to room temperature anytime you need it.