Q--When I pop microwave popcorn in the microwave, not all of it turns out; some kernels remain unpopped. The package directions warn me never to repop unpopped kernels. What`s going to happen if I do? Will the oven explode? And is there any real difference between regular popcorn and the stuff designed for the microwave?

A--Believe it or not, this question has plagued microwave oven manufacturers from Day One. Surveys show, they say, that an inordinate number of customers buy microwave ovens specifically because of their ability to pop corn.

And the unfortunate truth is that popcorn is a notoriously poor candidate for microwaving, because of the kernels` small size and relative lack of moisture. Microwave energy, you see, essentially locates objects (similar to the way radar ''sees'' objects) and transfers some energy into those objects, thus making them very hot. Microwave isn`t heat, it`s energy, and as such needs something with which to interact. ''The closest analogy I can give you is the old tree-falling-in-the-forest bit,'' says Robert Schiffmann, president of R.F. Schiffmann Associates, Inc., a New York firm that does extensive consulting and testing in microwave heating applications. ''Microwave energy needs to be absorbed by something to create heat.''

But microwave energy doesn`t ''see'' everything in the oven. Such things as glass, plastic and paper are essentially transparent to microwaves, and as such don`t get heated by microwave energy. (Yes, glass dishes feel hot coming out of the microwave, but that`s because of heat transference from the food they hold; the microwave doesn`t heat the dishes directly.) Microwave energy has a similar problem ''seeing'' popcorn kernels; they`re so small as to be nearly invisible.

If you were to put one kernel of corn into a microwave, it probably wouldn`t pop. The kernel would constitute what is called a ''low load'' in the trade. Put simply, there isn`t enough stuff inside the oven cavity to interest the microwaves. If you scattered a goodly number of kernels along the oven floor, you`d run into the same problem.

So some bright chap figured out that if you kept the kernels tightly bunched together, they`d form a mass that would be large enough for the microwave. And thus was born ''microwave popcorn,'' which is nothing more than popcorn kernels in a pouch, mixed with a slurry of oil and salt. The pouch keeps the kernels together, and the slurry, of course, does the flavoring for you.

But even then, the system isn`t perfect, and the reason should be obvious: As the popcorn pops, there are subsequently fewer and fewer kernels remaining to pop; the mass diminishes until it`s too small for the microwaves to pop the kernels.

As a result, there always will be leftover kernels when you finish popping. In fact, say manufacturers, the microwave popcorn packages are designed with a certain amount of waste in mind; they claim to put more kernels in the pouches than could be comfortably contained, if all were to pop; by popping what will pop and discarding what won`t, manufacturers say we`re assured of getting a full serving.

But ours is a perverse society. We discard major appliances when they get a little outdated, but, by golly, we`re not going to let a few valuable kernels of popcorn slip through our fingers. So some people continue cooking, trying to pop every last kernel, or gather up the unpopped kernels and try popping them later.

Both are bad ideas. Overcooking popcorn is unlikely to yield additional volume. What`s more likely is that the popped corn will present a more attractive mass to the microwave energy than will the unpopped kernels; all you`ll do is burn the good stuff.

Corn kernels, whether made in a microwave, skillet or electric popper, become popcorn by getting very hot very fast. This excites the moisture inside the kernel, rapidly building pressure so high that the kernel literally explodes. When a kernel is heated inefficiently (i.e, not hot enough or not fast enough), the moisture inside the kernel has time to escape. The moisture gets out (as steam), but the kernel doesn`t pop. This is why certain low-wattage microwave ovens make lousy popcorn; some of them simply can`t provide enough energy to get the kernels hot enough fast enough (corn pops at about 325 degrees) to achieve good results.

And reheating could be hazardous. No, the oven won`t explode, but you could start a fire or damage the oven interior by superheating dried-out kernels. The kernels, while not popping, could become hot enough to ignite, say, a paper container (which is why it`s a very bad idea to make a homemade popcorn popper from a paper bag) or damage the oven walls. Repopping popcorn isn`t always going to cause a problem, and you just might get a little extra popcorn for the effort, but why bother? Popcorn is so ridiculously inexpensive that repopping simply isn`t worth the risk.

As for the difference between popcorn and ''microwave popcorn,'' the basic difference is that microwave popcorn costs a lot more. The popcorn is identical; the price difference reflects the convenient, no-mess, no-cleanup packaging. In addition, popping corn in a microwave is tricky, and the prepackaged ''microwave'' dealies work pretty well. You can save money in the long run (if you eat popcorn frequently) by investing in a special microwave corn popper; on the other hand, Schiffmann reports satisfactory results simply from popping the stuff in a small glass mixing bowl covered loosely by a sheet of plastic wrap.