Q: Why does my gravy turn out to be either lumpy or greasy?

A: It doesn't have to be either lumpy or greasy. We all know people who can make it both lumpy and greasy at the same time, don't we?

Lumps and grease arise from the same basic phenomenon: Oil and water won't mix. In making gravy, you have to trick them into blending.

First, let's get some terminology straight. Oil, fat and grease are the same stuff. It's called "fat" when it's solid and "oil" when it's liquid. Any solid fat can be melted to a liquid, and any liquid oil can be solidified by cooling.

In their natural forms, fats are generally found in animals and oils are found in the seeds of plants. But food professionals call them all fats anyway, because that's the generic scientific name for the family of chemical compounds involved: glycerides of fatty acids.

"Grease" is an intermediate consistency between solid fat and liquid oil. The word has an unsavory connotation (a crummy restaurant is called a "greasy spoon"), and it is never heard at the dinner table except in the most dire of circumstances. In what follows, I'll use the words "fat," "oil" and "grease" as necessary to get my point across. Or frankly, I'll use whichever one I feel like using.

Let's talk about the most common kind of gravy: pan gravy made from the drippings of roast meat or fowl. Nobody likes thin gravy, so a thickening agent must be used. That's where flour comes in. Flour contains both starch and protein. Thickening a sauce with cornstarch or arrowroot, which are non-protein- containing starches, is a totally different ball game, so don't try to substitute them for flour in what follows.

When your turkey is done, remove it from the roaster and examine the awful-looking mess in the pan. You'll notice that there are two kinds of liquids: an oily liquid that is melted turkey fat and a watery liquid, the juices from the meat and vegetables plus any broth or water that you may have added. The trick is to incorporate both of these incompatible liquids into your gravy, because each contains a unique set of flavors. That is, certain flavors are fat-soluble and others are water-soluble. Your goal is to get the fat-based flavors and the water-based flavors to mix into a smooth, homogeneous sauce.

It's all in how you handle the flour, because flour is not only a thickening agent; it also does the job of blending the oil and water together.

Flour power

Flour is a very fine powder containing certain proteins that form a sticky substance-- gluten--when they absorb water. Now if you were to just dump some flour into the roasting pan and stir, the proteins and the water would get together and form a sticky glop. And since the glop is water-based, the oil wouldn't be able to penetrate it. You'd wind up with lumps of glop wallowing in a pool of grease. This may be standard fare in some households, but most experts agree that gravy should not be the chewiest part of a Thanksgiving dinner.

What should you do instead? It's as simple as one-two-three (plus two): 1) You separate the watery and oily liquids from each other in one of those clever gravy separators that pour from the bottom (the fat is the top layer, if you must ask). 2) You mix the flour into some of the fat. This blend of flour and fat is called a roux (pronounced "roo"). 3) You cook the roux a bit to brown it and to get rid of any raw floury taste. 4) Only then do you slowly stir in the watery liquids. The flour, oil and water will blend magically into a smooth sauce just as if they weren't natural enemies. 5) Finally, you simmer the sauce to break down the flour grains and release their thickening starches.

Oil and water do mix

Here's how it works.

By mixing the flour with fat first, you ensure that each microscopic grain of flour becomes coated with oil, so that the watery juices can't get through to gum up the flour's protein. Then, when you stir the juices into the roux, the flour grains become widely dispersed, taking their coatings of fat along with them. And that's exactly what you want: fat and flour uniformly dispersed throughout the liquid to make a smooth, homogeneous mixture. In short, you've persuaded the oil and water to fraternize by using the flour as a carrier of oil throughout the water. Then, when you simmer the sauce to let the flour do its thickening job, it does it uniformly throughout. No thick spots or thin spots. No lumps.

If you make your roux with too much fat, though, it won't all be picked up by the flour, and the excess fat will just hang around in greasy little pools, ruining your reputation. On the other hand, if you use too much flour, it won't all be coated by the available fat, and the extra flour will turn into lumpy library paste as soon as you add the watery liquid. So it's essential to keep the amounts of flour and fat just about equal.

How much flour, fat and watery liquid? To one part flour and one part fat, use eight parts of liquid juices and/or stock. (You will find detailed gravy-making instructions above.) Your gravy will be legendary.

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.