Find out why every time you accidentally leave a trace of tinfoil on your leftovers, your microwave looks like it's the Fourth of July.
For the lazy home cook, every day is a new adventure in package design. Take a new package out of the fridge, or a very old one out of the freezer, take a look at the guaranteed microwave-safe dish that will need cleaning if food is dumped into it, and think, "What the hell? I'll cook it in the package. There's probably no metal in there." Most of the time, this turns out to be a mistake. Sparks start leaping everywhere, and the period of nuking is over.
By putting metal in the microwave, what you're essentially creating is a localized lightning storm. A microwave works by sending an electromagnetic waves careening through its inner cavity. These waves are calibrated to excite water molecules, causing them to jiggle around heat up the food they're in. This is why microwaves work well on soup and not so well on ancient, dessicated bread. The water in a foodstuff (or small dog) heats up.
Although the electromagnetic waves in microwaves don't have much of an effect on any organic molecules, but all electromagnetic waves have an effect on metal. The outer electrons on a piece of metal are relatively free from the yolk of their respective atoms, and can wander around. When the waves hit them, they're pulled around quickly. Some are pulled between fluctuations of electrical potential throughout the metal, and accelerated through the air. They collide with atoms in the air, pumping in enough energy to their electrons to push them free of the air molecule. When those electrons fall back down to the molecules they left, they lose energy in the form of light. Those are the sparks you see behind the smoked glass of a microwave oven (and what you see in the sky during a lightning storm).
A lot of sparking can generate enough heat to set fire to the food, which will wreck the interior of the microwave, so get the metal out of there as soon as possible. The food and the metal itself should be just fine, though. There are no noxious chemicals. So chow down: It's no different from eating food that's been struck by lightning. Think of it as Thor food.