List of Things You Shouldn't Microwave

CDs and DVDs

As a general rule, if it's not food, it's probably better not to microwave it. However, you can get a cool plasma display and an interesting effect from microwaving a CD. The problem is, you could also get a fire, release toxic fumes, and ruin your microwave. Of course, the CD will never work again (although this might be a plus, if it's a Nickelback album). If the risk doesn't deter you, I have microwaved a CD and have some tips to minimize the risk.

Grapes

No, you don't get raisins if you microwave grapes. You get fire. Grapes are mostly water, so you'd think they would be okay. However, the roughly spherical shape of the grapes, combined with their waxy peel causes the microwaves to generate plasma. Basically, you get mini-plasma balls in your microwave. Sparks can jump from one grape to another or to the inner workings of your microwave. You could ruin the appliance.

Toothpicks or Matches

Standing up a toothpick or a match supplies the right geometry to produce plasma. As with grapes, the end result could be a fire or a damaged microwave. Actually, if you microwave matches, you're pretty much guaranteed that fire.

Hot Peppers

Do not be tempted to dry peppers using your microwave oven. Heating the pepper releases capsaicin into the air, which the microwave fan will disperse into the room and subsequently your eyes and lungs. There may be some value to this as a prank, since the risk to the microwave is minimal. Otherwise, it's one way to pepper spray yourself and family.

Light Bulbs

Why would anyone microwave a light bulb in the first place? The reason is because the energy emitted by the microwave illuminates the bulb. However, the bulbs also contain metal, so microwaving them generates sparks and unevenly heats the glass, typically breaking the bulb. Sparks and an explosion may result, so there is a good chance of ruining the microwave. If it's a fluorescent bulb, you'll release highly toxic vapors into the air, thus poisoning yourself. Do not microwave!

Eggs in Their Shells

It's perfectly fine to cook eggs in the microwave, provided they aren't still in their shells. Cooking an egg in its shell heats the egg faster than it can release pressure, making an egg-bomb. The best case scenario is a mess to clean up, but there is a strong possibility you'll blow the door off the microwave.

Water, Sometimes

You probably heat water in the microwave all the time. However, there is a significant risk of superheating water, which happens when the water gets hotter than its boiling point without actually boiling. When you disturb the water, it suddenly starts to boil, often explosively. People get burned every year, sometimes seriously, from superheating water in the microwave.

How can you avoid this? Ovens with a turntable prevent superheating by jarring the water enough that it should boil when it gets hot enough. Otherwise, don't heat water longer than necessary and avoid reheating water that you forgot about, since the air bubbles that help it to boil will have been driven off by the first go-round in the microwave.