Ever open up that box of brown sugar only to find it's as hard as a rock? It's probably happened to all of us and few things are as frustrating, especially when you're in the middle of a recipe.
Brown sugar normally has a soft and fluffy texture, almost like wet sand, owing to a moist molasses film. But when it's exposed too long to dry air, that moisture evaporates, causing the sugar to harden.
But those lumps of hardened sugar can be resoftened. If you can spare the time, seal the sugar in an airtight bag with something moist -- a slice of bread, a wedge of apple, or a piece of citrus zest -- for a few days to soften it.
When you need the sugar quickly, heat it up. Place the sugar on a baking sheet and warm it in a 200 to 250 degree oven until it starts to soften, several minutes or so. Or microwave it: Place the sugar in a microwave-safe bowl covered with a damp paper towel, or damp kitchen towel, and heat in 30-second increments until the lumps break up easily and the sugar softens.