While spoons are generally okay, forks can create sparks due to a built up negative charge in the tines. Depending on what else is in the microwave, the sparks can cause a fire. Even without causing a fire, these sparks can damage the inside of the microwave and make it unsafe to use.
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!
When you put metal in the microwave, the metal has so many electrons that will get pulled by the microwaves which causes a thin sheet of metal to heat up so quickly that it could burn the appliance. Metal with kinks in it are an even bigger risk.
Nothing bad happened. Microwaves bounce off of metal items but cause a slight eddy current on the surface so unless the fork was made of plastic with just a very thin metal coating or unless it is so close to the sides that it arcs or touches nothing bad will happen.
Basically, if you have a piece of metal in the microwave, charges in the metal move around. If there is a part of the metal that is very thin, such as with aluminum foil or a fork, a high voltage could build up that exceeds the breakdown voltage of air and cause a spark.
Metals conduct electricity with lots of electrons which move freely. Microwaves are absorbed by fat and sugar in foods. If metal is put into the appliance, the microwaves cause the electrons to move around, producing sparks and heat.
Since those minerals act like "tiny pieces of metal," microwaves bounce off them just like they would a fork, causing the sparking effect. The food is still edible after these incidents occur—it just doesn't taste as good since it's not cooked properly.
Sparking inside microwave is common and does not indicate that your microwave has gone bad. When a microwave sparks most people panic. But this situation is not to panic. It requires you to immediately switch off your microwave and perform the necessary inspection.
"When microwaved, all metals, including utensils, cookware, and travel mugs can spark a fire," says Shimek. This also includes any objects with metal in it, like Chinese takeout containers with metal handles, dishware with metallic paint or trim, or any packaging with metal twist ties or staples.
When the oven is empty none, or almost none of the microwaves are absorbed. A large amount of energy reflects around the oven chamber resulting in large standing waves that can damage the unit.
Separately, a spoon or fork are unaffected by the microwave field, (but always remember to provide a “load” in the oven that can absorb the energy). So a cup of coffee still with a spoon in it - no effect. A fork on a plate of reheated food - no effect.
Many people microwave travel mugs not realizing they're made of stainless steel -- which does not belong in the microwave. The stainless steel will not only block the heat from warming whatever's in your mug, but nuking it can also damage your microwave.
There is certainly no residual radiation of any sort and the food is no more likely to cause cancer after being cooked with metal around than had there been no spoon with it. In general, leaving a spoon in a cup of coffee or bowl of oatmeal isn't going to cause any trouble at all.
Bottom line, you're ok. Just don't make leaving metal in the microwave a habit. Technically it could short out or catch fire or otherwise start acting up.
Even with thick metal objects, it's safest to keep them away from your microwave.
Sparking microwaves are usually the result of metal fragments in the microwave or a problem with the waveguide cover. Both these problems are easily fixed. The next most common reason for a microwave sparking is a faulty diode.
The electric fields in microwaves cause charges to flow through metal. Thin pieces of metal like aluminum foil are overwhelmed by these currents, causing them to heat up so quickly that they can ignite.
A strong burning, oily smell or a little smoke is normal when using Convection BAKE, Convection ROAST or Combination Fast Cook for the first few times. The smell comes from the new convection heater and will go away with regular use.
You don't want to put carrots in the microwave. "Arcing is sparks produced inside the microwave from the microwave's reaction with metallic materials. Carrots may contain minerals from the soil that can cause this spark," she says.
If you aren't sure whether something can go in the microwave, check the back or bottom to look for a microwave-safe label or symbol. If you can't find one, try to determine what the item is made of—this will give you a better idea if it's ok for microwave use.
Marshmallows are mostly made of tiny air bubbles that are encased in sugar, gelatin and water. When you cook marshmallows in your microwave oven, the microwaves make the water molecules vibrate very quickly, which warms the air inside the marshmallows and causes it to expand.
''The microwaves are heating the water, which is the main constituent of wine,'' said Christian E. Butzke, an enologist at the University of California at Davis. ''If you do that for a very brief period -- 10 seconds maximum -- no other chemical reactions are going to take place, and nothing will be destroyed.