What can you add to dried out fondant to make it not dry?

Fondant that is dried-out or hardened can be placed in the microwave for several seconds to soften it. According to GlobalPost.com, glycerin can be added to fondant to prevent it from drying out.

Generally, commercial-grade fondant contains glycerin to prevent it from hardening. However, homemade fondant recipes do not always call for glycerin. In addition to adding glycerin to a fondant recipe, there are other ways to prevent it from drying out or hardening. When fondant is not being used, it is best to tightly wrap it in plastic wrap and store it in an airtight container at room temperature.