Popcorn is a specific type of maize (*Zea mays everta*) with a uniquely thick hull. Every kernel contains a tiny droplet of water stored inside a circle of soft starch. As the kernel is heated, the water turns into steam, creating internal pressure. The hard outer hull acts like a pressure vessel. When the temperature reaches about 355°F (180°C) and the pressure hits 135 psi, the hull suddenly ruptures. The starch inside expands rapidly into a white, fluffy foam that cools instantly into a crisp structure. If a kernel has a tiny crack in its hull, the steam escapes slowly and it won't pop—these are the 'unpopped' kernels found at the bottom of the bowl.