Why does popcorn pop?
Two factors combine to create the characteristic popcorn popping: efficient heat transfer to the interior of the corn kernel (the endosperm) and the high mechanical strength and integrity of the outer kernel layer (the pericarp).
Intense heating causes water within the endosperm to vaporize and this creates high pressure within the kernel. The pericarp, now acting as a pressure vessel, resists rupturing until a certain pressure is reached. At that point the kernel violently explodes, making the popping sound. When this happens, the starchy endosperm, expanding rapidly as its starch granules swell, creates the puffed texture of the now edible product.
Popcorn pops well only when its moisture content lies somewhere between 11 and 14%. The better brands are packed in airtight containers because their moisture level has already been adjusted to the required level. If the kernel is dry there will not be enough internal pressure to burst it. If the kernel is too moist the pressure will build up before the starch is cooked: the casing softens and cracks prematurely, and all you get is split kernels instead of fluffy popcorn.
Similarly, you will often get poor results if you cook popcorn gradually, on a low heat, rather than sudden high heat. It is the fast build-up of pressure--too rapid for the kernel to relieve gradually--that causes the split casing and the pop. Slow heating allows the kernel to soften, so it cracks at a lower pressure and allows the remaining water to escape early.
Not all varieties of corn will pop, however. Nonpopping varieties are those that have poor heat transfer by the endosperm and a weaker pericarp.
Source: The NewScientist - contributions by Julian South and David Hills