Popcorn kernels contain oil and water with starch, surrounded by a hard and strong outer coating. When popcorn is heated, the water inside the kernel tries to expand into steam, but it cannot escape through the seed coat (the popcorn hull). The hot oil and steam gelatinizes the starch inside the popcorn kernel, making it softer and more pliable. When the popcorn reaches a temperature of 180 °C (356 °F), the pressure inside the kernel is around 135 psi (930 kPa). This is sufficient pressure to rupture the popcorn hull, essentially turning the kernel inside-out. The pressure inside the kernel is released very quickly, expanding the proteins and starch inside the popcorn kernel into a foam, which cools and sets into the familiar popcorn puff.