Each kernel of popcorn contains a drop of water stored inside a circle of soft starch. (That's why popcorn needs to contain 13.5 percent to 14 percent moisture.) The soft starch is surrounded by the kernel's hard outer surface. As the kernel heats up, the water begins to expand, and pressure builds against the hard starch. Eventually, this hard surface gives way, causing the popcorn to explode. As the popcorn explodes, the soft starch inside the popcorn becomes inflated and bursts, turning the kernel inside out. The steam inside the kernel is released, and the popcorn is popped!