What actually happens if that fork you used to stir a container of leftover lo mein ends up accompanying the dish into the microwave?

More on Countertop Appliances

Bernie Deitrick, a Consumer Reports engineer, explains what happens when a metal object meets a microwave. “Microwave radiation can produce a concentrated electric field at ends, corners, or edges of conductive objects," he says, "which can heat the object at that point, ionize the surrounding air, or allow sparks to jump between points."

In plain English: Any of those effects can start a fire in your microwave oven.

To minimize risks, all metal object should be removed from items you're microwaving. Never use a metallic container, or dishes with metallic decoration, in a microwave oven. And generally, don't stray far from the microwave while it’s operating (just like you wouldn't stray too far from an oven as it bakes).

It’s also smart to do your best to keep the following items out of the microwave:

Nonmetal Sharp Objects

Is that Reuben sandwich you’re about to reheat stuck with a wooden toothpick?

“Anything that comes to a point can cause a spark and fire,” says Tim Jorgensen, an associate professor of radiation medicine at Georgetown University. Even sharp objects that aren't metal can set off the phenomenon known as arcing (sparks flying inside the microwave). And though all metal items are problematic, those concentrated to a sharp point—twist ties, for instance—represent a double whammy.

Give takeout food a once-over before you press Start.

Dirty Kitchen Sponges

Kitchen sponges—especially sponges cleaned in the microwave—are among the tops spots at home for harboring pathogenic bacteria and fostering microbe growth.

And while the microwave treatment—heat—is surely an effective method to kill bacteria in sponges, it probably won't kill all of them, according to Markus Egert, a professor of medical and life sciences, Institute of Precision Medicine, Microbiology and Hygiene Group at Furtwangen University in Germany and author of a study on this subject.

"The remaining [bacteria] grow up again and become resistant with time—and these resistant bacteria might be more harmful to humans," Egert tells CR.

If you do clean sponges in the microwave, Egert says to dampen them in soapy water first, adding that you shouldn't clean sponges this way more than a couple of times before tossing.

Your best option is to break out a new sponge.

Eggs in the Shell

Cooking whole eggs in the microwave is faster than hard-boiling them on your cooktop, but the time saved might not be worth it.

Eggs can explode in the microwave and even after you've removed them. This puts you at risk for a burn.

And it can even be a danger to your hearing. 

At a recent conference of the Acoustical Society of America, researchers from Charles M. Salter Associates, an acoustical lab in San Francisco, presented their findings on exploding eggs. In a test of nearly 100 eggs, 28 exploded with a sound-pressure range of 86 to 133 decibels from roughly a foot away. For context, 108 decibels is the sound pressure you might experience at a rock concert.

The noise from an exploding egg lasts only a few milliseconds, of course, while a concert exposes you to high decibels for an extended period. And hearing damage is less of a concern than the risk of burn from flying specks of hot yolk.

Still, when it comes to hard-boiling eggs, take the old-school route.