As anyone who's put marshmallows in the microwave knows, they expand a ton! Sometimes they puff up to literally more than twice their original size (YouTube video for those who haven't seen it).
At first I assumed it was because they had a lot of air in them, but that doesn't make sense. There's no way that amount of air can puff up that much from the heat!
What makes marshmallows poof up so much when they are microwaved?