Marshmallow in microwave

We were eating dessert with my roommate tonight, and decided to heat up marshmallows with chocolate.  When heated in the microwave, the marshmallow expanded instead of melting.  This is because marshmallows are made of foamed sugar. When it is heated, the water in the foam heats up and the trapped air in the foam causes the marshmallow to expand.  When the microwave stopped, the marshmallow collapsed because the air was no longer being heated.