The reason that we can’t put metal in the microwave is similar to why its a bad idea to run a microwave with nothing in it. If there is fat or water molocules to absorb and transform the microwave energy to heat energy then there’s no problems. However, if there is no absorbtion then the amount of radiation energy builds up in the microwave until it reaches a point where sparks can occur. This of course is a bad thing.

Source: Ask a scientist

During my searches for this info I also found Unwise Microwave experiments 😀 most of the things on there are just silly, but one piece of information was not only startling but also something I think that everyone should know (assuming its true).
If you heat water for a few minutes in the microwave, it is capable of actually reaching temeratures above 100 degrees C.
‘How?’ you ask?
Well its a bit difficult to explain but I’ll give it a go (much better explination on that site).
Water can only change to steam at a liquid/gas surface, in normal boiling (with an element or on a hob) the temperature is so extreme near these heat sources that seed-bubbles are created and they rise up through the water. In a microwave however, the water is in effect heating itself (by absorbing the microwave radiation) and so there is no localised hotspot for these seed-bubbles to be created. True there will be some micro-bubbles due to the un-even surfaces of the container that holds the water, but this will not be enough to cool the water (as with normal boiling).
Now, you have your superheated water and you take it out of the microwave, you add your sugar and…. KA-POW (not in a good way) with the introduction of all those micro-bubbles (and generation sites) the water will immediately equalise and rapidly cool by changing to steam. This is a pretty explosive reaction and will send boiling water all over the place, burning anyone nearby.
Not good !
So the advice is be damn careful and if you must re-heat coffee or boil water in the microwave, do it for less than a minute.