The rule is as simple as it is ubiquitous: don't stick aluminium foil in the microwave if you don't want to have to buy a new microwave. Everybody knows that, right? But, turns out, that old adage might be wrong.
Microwaves work thanks to magnetrons. This technology, originally designed for military radar systems, generates very short, high frequency radio waves from standard AC current—usually around 2.4 gigahertz. That frequency is just right for being absorbed by water, fat, and sugar molecules within organic matter. This extra energy causes them to vibrate at very high speeds, which in turn generates waste heat that actually cooks the food.
Contrary to popular belief, microwaves don't cook food from the inside out. Their radio waves actually only penetrate a few centimetres into food. It's the conductive properties of the meat and plant matter that heats it all the way through. The outside parts just carry the heat inside. That's why it's better to cook large pieces of meat at reduced power for longer periods—you ensure that the meat cooks all the way through without overdoing the outer bits. And since microwaves don't actually heat the air around the food, as conventional ovens would, food doesn't get crispy and golden. Which is occasionally a blessing, but most often a curse.
Inorganic materials are a different matter entirely. Non-reactive, non-metal vessels of ceramic, plastic, or glass do not absorb radio waves at that frequency and consequently, don't heat up very much in the microwave. Metal, on the other hand, actually reflects microwave energy. That's why the inside of every microwave is essentially a secure metal box—they keep microwaves from spreading out across the kitchen and cooking your insides from across the room.
Now things get a little weird. It turns out that any flat metal sheet—the thicker the better (and we'll get to why in a second)—can be used safely in the microwave because they act just like the flat metal walls: they reflect microwaves. A sheet of flat metal can actually be used to shield parts of your dinner to prevent them from overcooking since they would prevent the microwave energy from actually striking the food. And when placed under, say, a frozen pizza, a flat sheet of metal can actually help brown and crisp foods by reflecting additional energy against the bottom of the food.
Sometimes the electromagnetic field within the microwave can get a little mixed up and generate small arcs of electrical discharge. This can be caused by innocuous items like carrots (when grown in mineral rich soil) and hot dogs (when the salt and additives aren't properly mixed). More moderate sparking occurs with the gold paint on your nice dishes and forgotten twist ties. But a big hunk of curved aluminium foil like on a big box of wine spells instant appliance death.
Unlike the sturdy walls of a microwave, small, thin, and pointed pieces of foil cannot withstand the flow of microwave energy over them and rapidly heat until they ignite. Any thin, crinkled edges allow current to run along them, arcing against the microwaves metal walls and setting fire to your meal.
If caught immediately, you can avoid seriously damaging your microwave by simply removing the offending bit of metal. However prolonged microwaving of metal (or wadding up an entire roll and tossing it in a nuker) will likely cause irreparable harm to the appliance and probably set other parts of your kitchen on fire.
As such, follow the USDA's advice:
--Use new, smooth foil only. Wrinkled foil can cause arcing (sparks).
--Cover no more than 1/4 of the food with foil.
--Shape the foil smoothly to the food so no edges stick out.
--It makes no difference which side of foil (shiny or dull) is facing out.
--Do not place the foil closer than one inch from the oven walls.
--If the microwave oven has metal shelves OR a metal turntable, don't microwave food in foil containers or metal pans, and don't let foil used for shielding touch or be close to the shelves or turntable.
--If you see arcing (sparks), immediately remove the foil shielding; transfer frozen food from foil container to a microwave-safe utensil.
So as long as you keep the metal flat, thick, and covering as little floor space as possible, you will be able to throw microwave shade on your gourmet bachelor chow.