When the temperature around the kernel exceeds 100°C the water boils and water vapor reaches a thermodynamic equilibrium, like a pressure cooker. The starch granules swell and form a spongy layer and the corn 'POPS' and jumps.
That's why corn pops! Because of the water it contains and the pressure of its vapor.
Once it bursts the water vapor evaporates and the starch cools and solidifies into the final shape of the popcorn. The size of the corn kernel has increased tenfold.
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