The secret to crispy marshmallows is homemade corn syrup, a less stable version of what you buy at the store. Sounds like a bad thing, but it's not. You see, it's that very instability we need. During the candy making process, this syrup begins to crystallize due to its instability. This starts a chain reaction that ultimately results in "defective," crunchy marshmallows. Terrible for s'mores and Rice Krispie treats, but a thing of beauty for cereal.