Not a good idea, sparks will fly! I'm pretty sure you will damage the microwave as well if left any amount of time. I remember putting a saucer in the microwave not realizing it was decorated with gold. ... Ohhhhh pretty light show, actually kinda startling.

depending on the type of metal, the power of your microwave and how long the item is left 'cooking' will determine the results. Ultimately you will burn out your microwave, start a fire, melt plastic or shatter your glass door. A side note: Aluminum foil is safe to use in any microwave manufactured after the mid 1980's.

In addition to the safe and responsible answers that (accurately) predict disaster and mayhem, I'll add a bit more. To start with the unsafe and flip answer: In the most general terms, there is nothing fundamentally wrong with putting metal in the microwave, and the answer to whether you can put any in there is, as it often is in things like this, "it depends." Microwaves are just electromagnetic energy, and your microwave oven is designed to bathe whatever's inside it in that energy. So whatever happens to something inside a microwave oven is exactly related to how that something reacts to a strong field of electromagnetic energy. If you put a potato in there, the majority of the parts of the potato don't react at all to the electromagnetic energy and they just sit there. If potatoes were made entirely of that stuff potatoes would come out of microwaves just like they went in -- cold. Fortunately, however, there's a whole lot of water in potatoes, and water molecules spin in different directions in reaction to the electromagnetic energy. The friction from all those spinning water molecules (fat and sugar molecules work too) creates heat, and that heat bakes your potato. How about plastic, or anything else that's called "microwave safe?" Microwave safe is another way of saying "this doesn't care about electromagnetic energy" and so it just sits there. No water molecules to spin and heat up, nothing. It comes out like it went in. Now back to your question. How does metal feel about electromagnetic energy? Metal has a very specific reaction to electromagnetic energy. Everything from radio antennas to electric motors are based on the discovery that when metal is put in a changing electromagnetic field it causes an electric current to flow through the metal. And since most metals have some electrical resistance they also start to heat up as a result. So if you do the classic experiment and put some aluminum foil in the microwave (Don't. Seriously. Check out the links below instead) the light show that results is pretty straightforward: An electrical current is induced in the foil. That current wants to jump to other places in the foil, so you get sparks. The heat from the foil's resistance to the current is also potentially too high for the foil, so you can also get smoking and burning. The same goes for plates or cups with gold leaf: current and heat and sparks and smoke, all adding up to a much lower resale value. Put a fork in the microwave and you may get sparks between the tines. Put a heavy ice cream spoon in and you'll get current and heat, but no light show - there's no convenient place for the spoon to arc too and a heavy ice cream spoon usually has enough metal to carry the current without heating up too much. So once you understand the process you can pretty easily predict what will happen, just as I can predict that if you actually try any of this stuff you are going to end up with a wrecked microwave, burned forks, and a reputation for being not so bright. Besides, there's no need because in the grand tradition of the Internet, plenty of people have done crazy things for you already and were kind enough to post their movies and stories for you to enjoy from the safety of your bunker. A few microwave experiment links are below. Enjoy! http://apache.airnet.com.au/~fastinfo/microwave/ http://margo.student.utwente.nl/el/microwave/ http://www.everist.org/special/mw_oven/