When I was in fifth grade, my friends and I had this great idea to make a giant eraser out of a bunch of small erasers. It's not like we could just buy a giant eraser (we could). So we went about cutting up a bunch of normal-sized erasers. After a month or so, we had enough pieces of cut up erasers to fill a small plastic container.
Naturally, my friend Gaby takes it home and puts it in the microwave for a few minutes. Believe it or not, we made a giant eraser... but we also burnt a quarter-sized hole straight through the plastic container and probably almost started a fire in her house. You see, kids, there are some things you can and can't put in a microwave. About 75% of the time you cannot put metal in the microwave ... but what about that other 25%?
How Do Microwaves Work?
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The magic box takes your food and returns it warm, in exchange you get cancer 20 years later (totally kidding, I promise). According to the USDA, a microwave takes electric power from a wall socket and with a magnetron (magical device) converts the power to very short radio waves (aka micro waves, get it?). These radio waves are absorbed by water, fats, and sugars, which makes the molecules vibrate very fast and creates heat.
Contrary to popular belief, microwaves don't cook food inside out. The radio waves actually only penetrate food 1 to 1.5 inches into the food. In thick pieces of food, the radio waves don't reach the center, but instead of heated up by conduction of heat, which is the transfer of heat via direct contact.
Inorganic materials in the microwave work differently than food. Non-reactive and non-metal containers such as ceramic, glass, or plastic don't absorb the radio waves at that frequency. They don't heat up like your food does. On the other hand, metal reflects the micro waves of energy, and your microwave is lined with sheets of metal so that the waves are only in the tiny magic box cooking your food and not in the kitchen.
What Happens When You Put Metal in the Microwave?
Man put metal in magic box. Box made fire. Bye bye house.
There are actually two different answers to this question, because it the thickness and shape of the metal in the microwave to determines how it'll react.
Any flat metal sheet can be used in the microwave because it acts just like the walls of a microwave. It reflects the waves. Aluminum foil can be used to shield a piece of food so that it doesn't get cooked or it can be used to cook foods better such as Hot Pockets that have aluminum foil in the wrapping.
According to How Stuff Works, small, thin, and pointed pieces of foil (or metal, such as silverware) should not be put in the microwave. They cannot handle the energy waves and quickly heat up to the point of starting sparks and/or a fire. Any crinkled pieces of aluminum foil allow the waves to flow over it and create sparks that light a fire and destroy your microwave (and maybe your house if you aren't paying attention). Please don't do that.
#SpoonTip: The USDA has a few general rules for when using aluminum foil that I would highly recommend reading over before you try putting metal in your microwave. Honestly, I really think it would be smarter to just transfer your food to another plate if you don't want that one to cook or if the plate you're using is causing sparks (which means it has metal in it). It takes all of 10 seconds to change plates or start a fire in your house. Be smart.
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About two years ago, my friend's girlfriend put her iPhone in the microwave because she heard that it would charge the phone faster than normal. It did not (bye bye phone and microwave)... Again, there are some things you can and can't put in a microwave. Be careful when putting metal in the microwave—you've been warned.