Inside that is starch, which can turn into soft fluffy popcorn. Most of the moisture is in the starch. The water concentration is critical. But if the moisture level is too high, the pressure gets too high before the starch has been cooked - and in that case, the pericarp softens and cracks, and all you get is split kernels of corn, not soft fluffy popcorn. It's this fast build-up of pressure that splits the casing and gives you that wonderful pop.
Slow heating lets the pericarp soften, so that it slowly opens at a lower pressure.
If the water level and the temperature have been adjusted properly, as soon as the kernel explodes, the starch inside keeps on expanding to give you the soft fluffy texture of true popcorn. Not all varieties of corn will pop, but even so, there are 700 different varieties of corn that will pop. But even if you have the right sort of popping corn, and it doesn't pop, it could be because the moisture has been allowed to get too high or too low, or because the pericarp has been damaged. More Homework .