You can think of unpopped kernels of popcorn as little nuggets of stored mechanical energy, and that energy can be unleashed and transformed into force and motion when the kernel is heated. This is a very useful property, even if it's something that you can only do once, and the fact that popcorn is super cheap and not only biodegradable but also edible are just bonuses.
The "pop" in popcorn happens when enough heat is applied to vaporize the moisture inside the kernel. Over 900 kPa of internal pressure causes the yummy goo inside of the kernel to explode out through the shell, expand, and then dry. Relative to the size of the original kernel, the volume of a popped piece of popcorn has increased by a factor of at least five, although it can be much more, depending on the way the kernel was heated. Because of this variability, the first step in this research was to properly characterize the popcorn, and to do this the researchers, from Cornell's Collective Embodied Intelligence Lab, picked up some Amish Country brand popcorn (chosen for lack of additives or postharvest treatment) in white, medium yellow, and extra small white. They heated each type using hot oil, hot air, microwaves, and direct heating with a nichrome resistance wire. The extra small white kernels, which were the cheapest at US $4.80 per kilogram, also averaged the highest expansion ratio, exploding to 15.7 times their original size when popped in a microwave.
Mad, Tasty Science.