A popcorn kernel has an extremely hard hull that surrounds a mass of moist starchy endosperm (the food of the embryo). When such a kernel is heated to about 400 °F (about 200 °C), the moisture in the starch turns into steam and builds up pressure until the kernel explodes inside out into an irregular white fluffy mass about 20 to 40 times the kernel’s original size. About 25 corn varieties are suitable for popcorn.