Your microwave: It heats up your leftovers, pops your popcorn, and makes your old coffee slightly less awful. Chances are, you’re not using it as much as you used to.

But if there’s one thing we all know it’s that you should never, ever put metal in your microwave. Because it will definitely explode, just like in the clip above from the movie American Hustle. Except that scene is wrong. Remember that one time you absentmindedly left a spoon in the bowl you stuck in the microwave and everything was fine? Or whenever you've heated up a Hot Pocket and wondered about the metal lining in the packaging? That’s when you started thinking that the whole thing is just a hoax.

What's Up With ThatEach week, we'll explain the science behind a strange phenomenon that you may be wondering about, or may be hearing about for the first time right here. If you've seen or heard of something you'd like us to explain, send us an email.
Beer in a Hot Pan Slides Around Like an Air Hockey Puck
The Mysterious Effect That Makes Hot Water Freeze Faster Than ColdMicrowave ovens cook food using (what else) microwave radiation. Shortly after World War II, Percy Spencer, an engineer at Raytheon, accidentally discovered that the beam of microwaves coming from an active radar set had melted the Mr. Goodbar in his pocket. Spencer confirmed that it was the microwaves responsible for his chocolate mess by popping some popcorn and exploding an egg in the face of another scientist. Raytheon patented and built the first microwave ovens, which were so large and expensive they only ever got used on military ships. But by the 1970s the technology had gotten small and cheap enough for many people to buy kitchen-top models.

Inside the metal box of your microwave sits a device called a magnetron and despite its awesome name, understanding how this thing works is not exactly easy. The magnetron starts by releasing electrons from a negatively charged rod called a cathode. These electrons travel into a chamber filled with electric and magnetic fields, which deflect the electrons and force them to sweep around circles. As the electrons travel, they brush up against the positively charged walls of the chamber, which has special cavities built into it. The electrons zip over the cavities, imparting electric energy and forcing the cavities to resonate at a particular frequency that produces electromagnetic waves in the microwave range.

These microwaves are directed into the main box of your microwave oven, where they bounce back and forth between the walls. If you put something inside the microwave oven, it can absorb the microwave radiation. Certain substances – particularly water, fats, and other organic polymers – happen to be very good at absorbing microwave radiation. Water molecules do this by acting like tiny magnets (each molecule has a slight negative charge on its oxygen atom and positive charge on its hydrogen atoms). As the microwave beams zip back and forth in the oven, they attract the water molecules, tugging them and forcing them to vibrate very quickly. This vibration gets turned into heat, warming up your meal.

So that’s how to cook your dinner in the microwave. But what about metals? Well, at a microscopic level most metals are lattice arrangements of atoms with a bunch of electrons freely floating around between them. The microwave radiation will attract all these electrons as it bounces around inside your microwave oven, pulling them back and forth, and this generates heat inside the metal. A large sheet of very thin metal, like a big piece of aluminum foil, can in fact heat up extremely rapidly, becoming so hot that it could start to burn the microwave. So don’t do that.

But the real danger comes from having metal with kinks or dead ends in it. That’s because as the electrons get shuffled back and forth, they will meet up with other electrons. This can create concentrated spots of negative charge. Electrons will naturally be repelled from areas where there is too much charge. If these negative spots happen to find themselves in a place where they are near air, like in the tines of a fork or a kink in crumpled aluminum foil, the electrons will jump away, creating a spark and ionizing the air molecules into a plasma. Particularly awesome high-school physics teachers will demonstrate this effect to a classroom by placing a CD in the microwave and watching the sparks fly. The sparks themselves probably won't start a fire but if they hit something flammable, like wax paper that you also put inside the microwave, it can lead to a situation where you’re running for the extinguisher.

There are some situations where metal in the microwave isn’t going to be too dangerous. Thick metal heats up slowly. If it has no sharp edges, it isn’t likely to create sparks and fire, which is why some microwaves have a metal rack that you can place inside of them without any trouble. The metal lining of a Hot Pocket sleeve is also smooth enough to prevent sparking, instead heating up and generating infrared radiation that crisps up the outside of the food. And if the tines of the fork you accidentally put the microwave are buried under some food, the likelihood of sparks goes down.

While not all metal causes problems, one unexpected thing that can explode in your microwave is a grape (if it’s prepared correctly, like in the video below). Cut this tiny fruit in half, leaving the halves connected by the skin, and it’s about the same size as a single wave of microwave radiation. Because of their size and spherical shape, the grape halves are able to concentrate the microwave energy between them. The grapes will heat up, releasing steam at just the point where the microwave beams are most energetic. The steam will turn into a plasma (a high-temperature gas where the molecules are ionized). Within seconds, this will cause the grape to burst into flames. This is not an experiment to be taken lightly! The two grape halves can ruin your microwave.

So, as long as the metal you put in the microwave is thick, smooth, not near anything flammable and preferably buried in food (but not grapes), everything will probably be fine. But really, the best idea is still to never put metal in the microwave.

Videos: 1) _Sony Pi_ctures Entertainment/Youtube. 2) Ryan Shipman/Youtube