Marshmallows are a tasty, fluffy staple of summertime s’mores, campouts, and backyard barbecues. There really isn’t very much to them, either. The scientific way to see what really fills a marshmallow is to put it to the Marshmallow Masher test. You use a property of air to demonstrate what’s really on the inside of those roasted ‘mallows you make around a campfire. It’s not all sugar.