Q: When I make a meaty stock, soup or stew, it winds up with an oil slick on top--fat melted from the meat. I'd like to skim it off, but it's messy and I can never get all of it. Is there an easy way?

A: I know what you mean. You may have trimmed most of the fat from the meat, but your pot still looks as if the Exxon Valdez has run aground in it.

Recipes tell you to "skim the fat" as if it were as easy as peeling a banana. Supposedly, you just grab a spoon and scoop off the layer of fat without removing any of the underlying solids or liquids. But the word "skim" is a scam.

For one thing, it's hard to know how deeply to scoop without removing a lot of the underlying liquid. And if the pot or pan is wide, the fat will be spread out into such a thin layer that you can't remove it with a spoon. Moreover, there are probably lumps of meat and vegetables sticking up through the surface that impede your scavenging. And finally, if the bottom layer is a thick sauce, the oil might not even have had time to settle into a separate layer.

If there's not too much liquid in the pot, you can pour it all into a gravy separator--one of those glass or plastic cups that look like miniature watering cans and dispense contents from the bottom. The watery liquid will flow out, leaving the top layer of fat behind.

Or you can pour the slightly cooled liquids into a heat-proof glass container that is taller and narrower than your pot, so that the fat layer becomes thicker and can be sucked out with a rubber-bulb turkey baster.

The best method, if you have the time, is to put the whole pot in the refrigerator. But be sure to cool it first, so it doesn't heat the contents of your fridge to a dangerous, bacteria-friendly temperature. The fat layer will solidify and you can lift it off in pieces like ice from a frozen pond.

A wonderfully quick and easy method involves a midget-size mop--yes, a mop--that literally absorbs the fat. You just brush it across the surface of your soup or stew and it will soak up the oil without absorbing the watery liquid. It goes by various names, including Oil Mop and Grease Mop, and is available for about $6 at savvy kitchenware stores.

How, you ask, can a mop distinguish between an oily and watery liquid?

An ordinary mop absorbs water because the water wets--that is, sticks to--the fibers of the mop. There is an attraction between the water molecules and the molecules of the cotton, or whatever the mop fibers are. Moreover, water will even climb up between the fibers by capillary attraction, which I could explain to you but, after all, we're in the food pages. Anyway, when you dip an ordinary mop into water and withdraw it, a lot of water comes along with it.

But water doesn't wet all substances, by a long shot; water molecules just have too little attraction to certain other molecules. Dip a candle into water, for example, and it will come out dry. While water won't stick to wax or to many plastics, oils will. The Grease Mop is made of a plastic that is wetted by oil but not by water. The mop therefore sucks out only the oil.

Now that your mop is loaded with oil--and it can hold only so much per sweep--how do you dispose of that oil before the next sweep? If you live in a rented apartment and hate the landlord, you can hold the mop under hot water and let the oil go down the drain, where it will eventually find a cool spot and solidify, clogging the pipes beyond the reach of any plumber, short of tearing the building down. Alternatively, you can step out the back door and flick the mop smartly. A little shower of oil won't hurt the grass and it's biodegradable. (The ants will even thank you for it.) Then, back to the kitchen to sweep and flick again, until all the fat is gone from your stew.

Q: I use only microwave-safe dishes in my microwave oven, but some get a lot hotter than others. Why?

A: "Microwave safe" doesn't mean that a dish won't get hot. It means that it won't get hot from the direct absorption of microwaves. But the food it contains does absorb microwave energy and therefore gets hot, and much of that heat is transmitted to its dish. How hot the dish gets depends on how efficiently it absorbs heat from the food, and different materials--even different "microwave-safe" materials--can vary quite a bit in that respect. Be careful when taking any microwaved dishes out of the oven, because you never know how hot they will get.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking questions to wolke@pop.pitt.edu.