When a seed of popcorn is heated up in oil, it pops like this:
You can take one of these popped pieces and eat it with little to no problem. However, if you get an un-popped seed and sink your teeth in, it is noticeably harder. Why is this?
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Normally I feel uncomfortable giving Wikipedia-based answers since it seems lazy, but the Wikipedia article does a good job explaining it. It appears that the heating itself, rather than the popping, causes the popcorn starch to soften:
Under these conditions, the starch inside the kernel gelatinizes, softens, and becomes pliable. The pressure continues to increase until the breaking point of the hull is reached: a pressure of about 135 psi (930 kPa) and a temperature of 180 °C (356 °F). The hull ruptures rapidly, causing a sudden drop in pressure inside the kernel and a corresponding rapid expansion of the steam, which expands the starch and proteins of the endosperm into airy foam.
Additional evidence that it is the heating, and not the popping, which causes this softening is given in the following citation:
If heated too quickly, the steam in the outer layers of the kernel can reach high pressures and rupture the hull before the starch in the center of the kernel can fully gelatinize, leading to partially popped kernels with hard centers.