Granulated sugar will harden when exposed to any type of moisture.
Here is an easy way to make hard granulated sugar soft:
1. Preheat oven to lowest temperature, 150-200 degrees.
2. Remove sugar from the package and put in an oven-proof container that will hold the sugar.
3. Place in warm oven for approximately 15 minutes.
4. Tap sugar with a spoon.
5. If it starts to fall apart, turn oven off and leave sugar in the oven for an hour or two to completely dry out.
Store in an airtight container to keep sugar free flowing.