Even the most cooking challenged would agree: you'd be hard-pressed to mess up popcorn, a TV dinner, or soup in the microwave. But stick a sheet of foil in on accident, and it's a recipe for disaster — just ask my 8-year-old self! So why does the microwave take poorly to metal?
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.