Ok, I guess this one comes to me. The inside of your microwave is made of metal because it reflects all that microwave energy into the food inside. The electromagnetic energy is produced by a magnetron that is tuned to oscillate at 2.45 GHz, the natural frequency of oscillation of the water molecule. You set up an environment with lots of bouncy energy, and the water inside of the stuff inside naturally bounces at the same rate, and by vibrating, heats up the stuff.
That's why you can't heat anything in a microwave that doesn't have water in it. Also and as well, metal stuff inside can reflect that microwave energy in ways that don't work for the unit. This can cause sparks and flares and other cool stuff. Throw a CD inside for giggles and ships.
I hope this answered your question, O Small Furry One. Feel free to ask again, I can make up all sorts of answers.