Microwave ovens are using electromagnetical fields of high energy generated with a device called magnetron.
Simply spoken, a magnetron is a very powerful radiotransmitter. In the case of a microwave oven it works with a frequency of around 2.4 GHz (= 2400 MHz).
The electromagnetical waves sent off by the magnetron brings the molecules of the water within food into a very fast oscillation - this produces the heat which we are using in a microwave oven to heat up food.
As only the water (within food) is brought into oscillation, other stuff like microwave-dishes isn't heated when within a microwave oven.
As high energy electromagnetical fields allow to do some other things than heating up food, I sometimes playing around with them to produce some interesting effects... :-)
Never ever every try to decouple the microwave field by using a wire to the outside of your microwave oven! This for sure would have dramatical effects to your health!!
Again: Don't do play around with microwave ovens - those aren't toys! If you don't know about the consequences and how to probably cope with them, keep off your fingers!
If you really think you have to know something special, please ask me. I either can explain it to you or even do the documented test here myself.
Please also note that some of my experiments could overload and destroy a "conventional" microwave oven. My one is a bit prepared to not having this problem. Don't ask what I did for that - I won't tell you anyway... ;-)