Popcorn is a specific type of corn (*Zea mays everta*) that is the only variety that pops. Each kernel contains a tiny droplet of water stored inside a circle of soft starch, surrounded by a hard outer shell (the pericarp). When heated, the water turns into steam, creating pressure inside the kernel. At approximately 180°C, the pressure reaches about 135 psi, and the hull ruptures. The starch inside expands rapidly into a white, fluffy foam which then cools and solidifies. Native Americans were the first to discover this, and archaeologists have found popcorn remnants in New Mexico dating back thousands of years.