If you've ever accidentally left a fork on the plate of food you're heating up, you know that metal and microwaves don't play so nicely together. Mini-lightening bolts are pretty, to be sure, but a bit frightening when they're in your kitchen. Ever wonder why in the world this happens?
This question proved a bit trickier to answer than we had first supposed! Microwave ovens work by electromagnetic radiation. A transmitter inside the microwave (like a mini radio-tower) sets up an electromagnetic field, which sends out microwaves of radiation.
These microwaves hit the water molecules inside your food and make them start wiggling around. This motion creates heat, and voila! Your food gets hot!
Part of the problem with metals is that they are a lot more solid than food. As food gets hot, water just converts into steam and the energy gets released. As the molecules inside a piece of aluminum foil heat up, they don't have anywhere to go. The metal heats up very rapidly and will eventually catch on fire. By the way, some manufacturers of packaged foods take advantage of this by wrapping their food product in a layer of foil that gets just hot enough in the cooking time to help the food to cook evenly and completely. Anyone have a Hot Pocket lately?!
Depending on their shape and surface smoothness, some metal objects will reflect the microwaves instead of absorbing them...which then bounce off the metal in the walls...which then bounce off the object again...and so on until eventually the transmitter itself overheats and becomes damaged.
Another problem is that metals are electrically conductive. If the object has pointy bits, like a fork or the metal filigree on a fancy plate, electrons collect at the edges. When enough electrons build up, an arc can jump between the metal and the electromagnetic transmitter. This is what creates that mini lightening show in your microwave!
In summary, don't put metal in a microwave. You're not going to burn your house down, but you could do damage to your microwave and give yourself a good scare in the process!
Any scientists in our midst who want to correct our amateur science or shed more light on the situation?!
For more information:
• Why No Metal in Microwave Ovens? from Suite 101
• Why Can't You Put Metal in Microwave Ovens? from McGraw-Hill's Access Science
• Metal in Microwaves from CalTech
Related: Fact or Fiction? Exploding Water in the Microwave
(Image: Flickr member Stan licensed under Creative Commons)
10 Picnic-Ready Salads to Eat Outside
Easy Entertaining Recipe: Grilled Peppadews with Smoked Mozzarella
Good Food to Share 
The Big Book of Small, Cool Spaces 
Apartment Therapy Presents 
The Eight-Step Home Cure 
Red-and-Pink-Stripe...
Must-See Small Cool Homes: Week Two
Juan's Garage Turned Modern Guest House
When I saw this, I immediately thought of Alton Brown's brown paper bag microwave popcorn. He has you staple the bag twice, and says that staples are safe in the microwave because they are shorter than the wavelength of the microwaves - they won't heat up.
Plain Brown Popper Recipe
I've used this recipe for years, without any problems. But I have to admit that I feel nervous every time I use it.
i have a related question - why does my food keep sparking/starting on fire in the microwave? is it time to get a new one? whenever i try to reheat cubed veggies that were tossed in olive oil and sea salt and roasted on parchment paper or in a glass baking dish, i put them on a microwave safe plate (i have tried a variety) and they start sizzling and sparking after only 2 second or so. it happens with potatoes, sweet potatoes, butternut squash and broccoli.
any ideas? is my microwave shot - is it time to get a new one?
@kategal25- this happens to me when I nuke a dish that has dried and/or fresh herbs. Could that be it? I don't know why it happens though.
@kategal - I'm going to posit a guess and say this is because of the salt & herbs. Same thing would happen if you put a sheet of paper in the microwave.
Microwaves work best on water, and when there isn't water stuff heats up far too swiftly and catches fire. This is why the dried herbs are getting drier.
Salt is a crystaline structure and also conducts electricity VERY WELL! So it generates the little edge electrons very efficiently - thus the sparking.
Try to nuke your veg without the salt and see if it does the same thing - if you must have the salt, add it afterwards or make sure to give the whole thing a good stir before placing it in the microwave so the salt bits are adequately moistened.
My grandmother would always keep a measuring cup of water (small) in the microwave to prevent it from breaking itself if it were ever turned on while empty.
The salt is what does it. Basically the salt is acting like metal in that circumstance. Best to stir your salt into the food first or add it after cooking. By the way NEVER try to reheat sauerkraut in the microwave, that really throws up sparks.
By the way if you have an old microwave you don't mind messing up throw an old cd in it. It's quite fascinating. Make sure you don't want the cd ever again, used to do this in highschool with AOL cds.
ahhh... makes sense - thanks!
Here's a little known trick, you can actually put a metal bowl in the microwave and nothing will happen. I'm not totally sure why but we do it at my bakery. Something about the shape being round and not having a place to release the energy.
BUT: Don't let the metal edge of the bowl hit any part of the microwave, if it does than the metal to metal connection is a point where the energy can be released.
Seems impossible, but it works.
When all the cooking and science experiments are done, I leave a small plastic dish half-full of baking soda inside the microwave. Just like in the fridge, it helps absorb any lingering odors.
PS, my microwave is not very smart: after I boil a cup of water it says, "Food is ready".
My microwave came with metal racks. I am still confused by this and have never used the microwave with them.
My MW also came with a metal rack. It is for use with the convection setting. It raises the food dish so that the hot air can circulate all around the food.