Dear Lou placing a spoon in the microwave (voiding the warranty?)
During cooking, the cooking chamber of a microwave oven is full of intense 2.54 GHz microwaves-electromagnetic waves with a wavelength of about 12.4 cm. Like all electromagnetic waves, these microwaves consist only of electric and magnetic fields that endlessly recreate one another as they propagate through space.
When the microwaves encounter the aluminum foil, their strong electric fields push around the mobile electrons in the aluminum. Those electrons often accumulate at the sharp tips of the foil, where they are prone to jump off into the air as a corona discharge. The air around the tips ionizes as the electrons enter it and current can begin to pass through the air itself as sparks.
This sparking around sharp points is one of the reasons why you are warned never to put metal in microwave ovens. The other reason for the ban is that thin pieces of metal can overheat as the microwaves propel currents through them, thereby starting fires. But metal isn't always a problem in microwave oven. A thick, rounded item such as a good-quality stainless steel spoon doesn't cause any significant problems. I leave such spoons in my coffee or tea frequently. While the spoons rearrange the microwaves slightly, they neither spark nor overheat.
Answered by Louis A. Bloomfield of the University of Virginia