Why does popcorn pop?

A popcorn kernel is a tiny pressure cooker. Inside its hard shell is a little bit of water and a soft starchy centre.

When you heat the kernel, the water turns into steam. The steam has nowhere to go, so pressure builds up inside the shell.

At about 180 degrees, the shell can no longer hold the pressure. It bursts open. The hot starch rushes out and puffs up into the white, fluffy shape — and the escaping steam makes the pop sound.

Only one kind of corn can do this: its shell is strong enough to trap the steam. Kernels with a cracked shell or no water inside never pop — those are the ‘old maids’ at the bottom of the bowl.