It is said that there are spirits who lived inside each kernel of popcorn. The spirits were quiet and content to live on their own, but grew angry if their houses were heated. The hotter their homes became, the angrier they'd get - shaking the kernels until the heat was too much. Finally, they would burst out of their homes and into the air as a disgruntled puff of steam.
Though there are no spirits that we know of in popcorn, each kernel of popcorn does contain a small drop of water stored inside a circle of soft starch. (That's why popcorn needs to contain 13.5 per cent to 14 per cent moisture.) The soft starch is surrounded by the kernel's hard outer surface. As the kernel heats up (to about 200c), the moisture in the starch turns into steam and builds up pressure against the hard starch. Eventually, this hard surface gives way, causing the popcorn to explode. As it explodes, the soft starch inside the popcorn becomes inflated and bursts, turning the kernel inside out, and increasing the kernel up to 40 times its size. The steam inside the kernel is released, and the popcorn is popped!