four major components: hull or pericarp, endosperm, germ and tip cap. 14 percent water. When you open a bag of freshly popped popcorn, you may see some of the water from inside the kernels. The water escapes into the air as a puff of steam. When a kernel of corn is heated, the water inside expands and creates pressure against the kernel's hard starch surface. This pressure eventually causes the hull, or pericarp, to break, and the soft starch inside the kernel becomes inflated and bursts.