This is an instruction that is often repeated to children: you should never put metal objects, such as cutlery, in a microwave oven.
An imprudence that causes the appearance of large sparks, electric arcs, or even flames, and can therefore be dangerous for the user, as well as for the circuits of the machine.
This phenomenon is caused by the electromagnetic waves produced by the device. Their precise frequency (2,450 MHz) agitates the metal molecules, as well as those of the food, which makes it possible to heat them.
And besides being perilous, putting a metal dish inside a microwave turns out to be perfectly useless. The waves are indeed reflected by this material, and can not, therefore, reach and warm food.