Why does popcorn pop?

Answer: Water vapor builds pressure

Heat breaks the kernel — Wrong. Heat doesn't directly break the kernel. The shell is tough and can withstand high temperatures. What breaks it is the pressure buildup from water turning into steam inside the kernel.

Water vapor builds pressure ✓ — Correct! Each popcorn kernel contains about 14% water inside a hard shell. When heated, this water turns to steam and builds up pressure. At about 180°C, the pressure reaches 9 times normal atmospheric pressure and the shell ruptures. The starch inside instantly expands into the fluffy white foam we love!

Air expands inside — Wrong. There's no air pocket inside popcorn kernels. The pressure comes from water molecules turning into steam vapor. This phase change from liquid to gas creates much more pressure than air expansion would.