Since you can't see microwaves -- can't feel them or smell them, either -- they naturally begin to be thought of as magic, both dangerous and wonderful at the same time.
Human beings seem to adjust so easily to the idea of magic, first getting used to the idea that unseen electromagnetic waves can cook their food, then becoming more and more demanding ("okay, you can bake a potato in five minutes, let's see you cook this meatloaf in 30 seconds"), and finally downright petulant when they discover that there are some things microwave ovens just can't do.
Most of the misconceptions about microwaves are born of their seemingly magical powers. Microwave ovens work by sending out electromagnetic waves which agitate the molecules in the outer inch or so of the food or liquid. This agitation produces heat, which then travels by conduction to the center of the food or liquid, just the way it would in a conventional oven.
But then things get a little spooky. The oven itself doesn't get hot; you can reach in directly after opening the door and feel no heat. Dishes that hold the food don't get heated by the microwaves because they contain no water molecules. If they do get hot it's only because heat has transferred from the food.
Then there is the excitement that can be produced by putting metal into a microwave. Microwaves are repelled by metal, so in some instances -- but not all -- you can get a sideshow of sparks and sputtering known as "arcing." On the other hand, very shallow metal dishes, such as those used with frozen dinners, work just fine, as does aluminum foil used as a protector against hot spots. See what your manufacturer says about metal; microwave ovens differ.
The one thing that is certain is that microwaves don't penetrate metal, so cooking will be much slower in places where there is a metal cover.
Since the method seems to be so mysterious, it's only logical to assume that there would be all kinds of special requirements for utensils used in the microwave. Actually, any dish will do that doesn't have metal trim (which sometimes causes arcing and sometimes darkens and causes the dish to crack). I've never found one of my own dishes that didn't work. Glass, pottery, china all work well. If you want to make sure, put the dish in question in the oven and turn it on high power for 30 seconds. If the dish doesn't get very hot, you're okay.
You can even use plastic or paper utensils if you're just heating things up, but plastic will often get too hot and melt (from long exposure to very hot food) if you use it for cooking that takes more than a few minutes, and paper sometimes falls apart.
In other words, it's not at all necessary to rush out and buy oodles of special dishes when you buy a microwave. Some are very convenient, it's true, especially those that can go from freezer to microwave, but it makes sense to get used to your new toy before you invest in lots of special dishes to go with it.
The same goes for coverings like waxed paper, paper towels and plastic wrap. Regular works.
Another misconception about microwave cooking is that everything goes faster in a microwave. Many things do cook faster -- a lot faster -- in a microwave, and that's what it's in business for. But don't expect across-the-board miracles; some processes are actually slower in the microwave.
Cooking frozen vegetables, for example. If you time yourself, you'll find that top-of-the-stove cooking is usually a couple of minutes faster. Directions for microwave-cooking instruct you to let these vegetables stand, covered, for several minutes because they continue to cook after they've been removed from the oven. If you serve them without the standing time, you'll find that some pieces are cooked and some are cold.
Cooking fresh vegetables in the microwave can save you lots of time in some cases, but actually take longer in others. Whole heads of cauliflower will be done in just a few minutes, as will baking potatoes. But pieces of these vegetables will often take about as long as whole versions. Fresh spinach, which takes just a minute or two on top of the stove, takes between 8 and 10 minutes, including standing time, in the microwave.
Also, in microwaves, cooking time multiplies as amounts get bigger. One baked potato takes about five minutes -- an extraordinary convenience -- but four take about 25 -- still a convenience but not quite so extraordinary.
There is another microwave myth holding that microwaves work from the inside out so that thickness of the food doesn't matter. Not true. Only the outer inch or so of any food is cooked by microwave magic. Interiors cook just like they do in conventional cooking. In other words, the larger the surface area (in relation to the mass) exposed to the microwaves, the faster the dish will cook.
Thickness also affects the speed with which a microwave will defrost foods, one of the things it's best known for. Far from being instantaneous, however, defrosting certain things can take a while. A five-pound duck, for example, will take about an hour, including 10 or 15 minutes of standing time. A four-pound roast will take about an hour, maybe slightly longer. A pound of hamburger, on the other hand, will be defrosted in about 15 minutes, including standing time.
The process itself, while certainly worthwhile and some would say even worth the price of the machine, is also not perfect. The outside of food will often begin to cook before the inside is thawed. To eliminate this problem, you can cover too-warm parts with aluminum foil. Don't let the foil touch the sides of the oven, however.
Microwaves are terrific at reheating foods without drying them out, so they have a reputation as the best vehicle for heating bread. In some cases this is true. Tortillas, for example, after a few seconds in the microwave are hot but still pliant. But other kinds of bread can get hopelessly tough and chewy in the microwave, especially if you overdo it by a few seconds. Thawing and heating a frozen whole loaf is particularly disappointing; unless you use very low power (and therefore quite a bit of time) you're apt to end up with a mass too tough to eat.
So stick to what your microwave is good at (visit any microwave store, read any magazine microwave cooking column, and these things will quickly become clear) and don't expect it to work any additional miracles.