Why does a microwave cooker heat up an empty plate which is clean and dry and microwave proof?
Mainly because the fact that a microwave only excites H2O molecules is a misconseption. A microwave attemps to excite all molecules, but only those that have a natural frequency close to the microwave frequency with resonate, and therefore excite like none other. H2O is one of those molecules, but there are others that can get heated up by microwaves, maybe less efficiently, but still. The difference between the microwave plate and your own plate is probably subtle, but enough. Microwave proof doesn’t necisaraly mean that the microwave can’t heat it up. It more or less means the microwave can’t damage it.
