The answer is water. The popcorn kernel's general composition is carbohydrate (principally starch), protein, fat and water. Water is stored in a small circle of soft starch in each kernel. As the kernel is heated, the water heats, builds up pressure and takes up any available room. The harder surface surrounding the starch resists the vapour pressure for as long as it can. When the outer surface gives way, the water vapour further expands, causing popcorn to explode. The soft starch pops out, the kernel turns inside out, steam inside the kernel is released, and the corn pops.