Originally Posted by
Verdris
Contrary to popular belief, you CAN safely put metals in the microwaves, but only certain shapes. Metal in the microwave produces catastrophic results when the object has any jagged corners or sharp edges. The EM emitted by the magnetron induces a current in the object, and where there are sharp edges or corners, current tends to gather there, creating a potential difference (voltage) that is discharged when it reaches a certain level. Thin, jagged items such as a crumpled ball of aluminum foil are quite dangerous, since it doesn't take much to accumulate a proper voltage, and the charge can freely jump from crease to crease. Microwave a fork, and you see great arcs run between the tines.
However, a smooth metal sphere can be microwaved quite safely, and you could probably get away with nuking a spoon, too.