How/why does popcorn pop?

Answer 1

When a popcorn kernel heats up (either in a popcorn popper or in the microwave), the moisture inside the kernel expands. Moisture is extremely important to a popcorn kernel. Unless the percentage of moisture in the kernel is just right, the kernel won't pop. When the pressure inside the hard shell gets high enough, the kernel explodes.

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Answer 2

Heating the kernels causes moisture within to become steam...steam is far less dense and since there is no additional volume for the steam to be...POP!