The key to why popcorn pops is its unusual moisture-proof hull. As the kernel is heated beyond the boiling point, the water inside begins to turn into steam and expand. Since the hull will not let steam out, the pressure inside the kernel begins to rise. The hull can handle a pressure of around 135 psi before bursting open. At this point, the pressure inside the kernel is released very rapidly, expanding the starch and proteins into a dense foam that sets quickly.