If you have marshmallows that have gotten old, stale, and hard all you have to do to make them soft again is to place them in a resealable plastic bag with a slice of bread. Let them stand until soft, which usually takes 1 or 2 days. Then you can remove the bread and the marshmallows should be soft again.
What a great hint! I had never heard of this so retrieved two small dabs of rock-hard mini marshmallows from the back of the pantry and tried this idea. It worked! I can tell already with just an overnight try that this is going to work beautifully. Thanks!