When a microwave oven stops, why are unpopped kernels very hot and popped kernels not hot? A commonplace empirical observation is that when a microwave oven stops, unpopped kernels are very hot (it's physically painful to touch them) and popped kernels are not.
Is there an elementary (or not) exposition of the physics involved?
 A: Popcorn pops in a microwave oven due to the microwaves interacting with the moisture in the popcorn kernel raising its internal temperature and pressure. Once the pressure increases enough the kernel pops and the moisture escapes and cools. The moisture in the un popped kernel remains hot.
Hope this helps.
A: How hot (or cold) something feels is not just down to temperature, but to the rate of heat transfer, or thermal conductivity. Popped corn is a good insulator, having a foam-like structure. As a result, little heat escapes to the hand. Unpopped corn is a much better conductor of heat, and transfers heat to the hand much more quickly, so that it feels much hotter.
Similarly, at normal temperatures, a metal may feel cold, because it conducts heat away from the hand. Expanded polystyrene or wool at the same temperature feels warm because it insulates against heat loss.
A: Winging it:
Microwaves heat the corn kernel which behaves like a tiny pressure cooker. Due to increased pressure, water inside will remain liquid at higher temperatures than at atmospheric pressure. So, temperature and pressure rise.
If it doesn't pop, then... well, it'll stay hot for a while.
If it pops, the pressure vessel is broken, so pressure inside the kernel drops to atmospheric. With dropping pressure, boiling point of water drops down to 100°C, thus the water is much hotter than its boiling point (ie, superheated) so it flashes to steam. This creates a BLEVE (Boiling Liquid Expanding Vapor Explosion) and makes lots of steam bubble inside the kernel, which expand and turn it into foam, ie popcorn. It is no different from a boiler explosion.
The amount of steam generated depends on temperature. All the water may not boil, some may remain liquid. In order to turn into steam, water needs an amount of energy known as Enthalpy of vaporization that is taken from the thermal energy. This means temperature drops until water stops boiling. When water boils, the steam takes away energy, not just its own latent heat but also its enthalpy of vaporization. It will return this energy when condensing on an object (since we're in the kitchen, think steam cooking broccoli).
Anyway. When they pop, kernels release steam that takes away the latent heat of vaporization, but it doesn't stop there. The popped kernel is still hot and has lost its shell, so any water that hasn't turned to steam will tend to evaporate, further cooling it. On the other hand, unpopped kernels didn't release any steam, and water can't evaporate through the hard shell, so all the heat is still trapped inside.
If you measure the temperature with your finger instead of an infrared thermometer, then what you're measuring is how much the object is heating your finger. If the object has low thermal mass and low thermal conductivity compared to your finger (like foam or a popped corn) then.. if it is hot your finger will cool the surface and take away very little heat from it, which you won't feel as hot. The core may remain hot, but you won't feel it.
