Microwave oven transmitters produce electromagnetic radiation ("microwaves") that disturbs electrons (the negatively-charged bits inside atoms), creating kinetic energy and thereby producing heat. That's how materials that receive radiation, like food and water molecules, become heated. The game changes with certain metals, however, which can be so dense that molecules, when agitated, have nowhere to go. The electrically conductive metals bounce off microwaves, rapidly creating a high voltage between the metal and the radiation source. Once the voltage strength surpasses a certain threshold, a spark is produced and can lead to a fire.
Have you ever unknowingly put metal in your microwave? If so, tell us what happened.