It's back-to-school time, and this is our first short-answer, pop quiz of the semester. Except that in this case it's the students quizzing the professor.

Here are a couple of my readers' questions that can be answered in less than an entire column's worth of space.

In your book, you claim that confectioners' sugar goes cloggy in cold drinks because there is cornstarch in confectioners' sugar and that cornstarch does not dissolve in cold liquids. However, any decent chef knows that in order to add cornstarch to a sauce you must first dissolve it in cold water because it makes cloggy lumps if added directly to a hot liquid. Can you please clarify this point?

I love your word, "cloggy."

First of all, starch doesn't actually dissolve in either hot or cold water as, for example, salt does. It merely disperses itself throughout the liquid like so much mud.

Cornstarch exists in the form of granules, tiny sacks of gummy starch. In hot water, the granules absorb water and swell until the sacks burst open, spilling their gummy contents. That's how starch thickens a cooked sauce.

But if you were to add the starch directly to the hot liquid, some of its granules would swell and burst immediately, gluing other granules together and forming clumps. The granules that get trapped in the interior of a clump never get a chance to get wet, swell and burst, so the clumps remain clumps: raw, dry starch in sticky coatings.

In cold water, on the other hand, the granules don't absorb water as readily. You have plenty of time to stir or whisk them around, dispersing them uniformly throughout the water. You can't do that very well in your glass of iced tea, but if your objective is to thicken a sauce, you can stir the starch briskly into a small amount of cold water. The dispersed granules are now suspended throughout the cold water. Then, when you pour them into your hot sauce, the granules can go into their their swell-and-burst routine independently and homogeneously throughout the liquid. No lumps, clumps, hunks, chunks, blobs or clogs.

Recently, I've noticed new types of margarine containing "palm fruit oil" and "fractionated palm oil." They advertise "no hydrogenated oils" and "no trans fatty acids." I thought palm oil was one of the "bad fats." Is there really a difference between palm oil and palm fruit oil? And what about palm kernel oil, which I see on other ingredient labels?

Confusing, isn't it? Let's start at square one.

There are two types of edible vegetable oils: oils from seeds and oils from fruits. The seed oils include canola, corn, cottonseed, peanut, soy, safflower, grape seed, and so on. The four common fruit oils are olive, coconut, palm, and palm kernel. The last two come from the oil palm, Elaeis guineensis, with Malaysia being the major producer and exporter of its oils.

Palm oil comes from the pulp of the tree's fruit, while palm kernel oil comes from the two or three kernels found inside the nut of each fruit. The designation "palm fruit oil" is probably meant to indicate the pulp oil (palm oil), rather than the kernel oil. Of course, the word "fruit" arouses pleasant, healthful vibes in the consumer, so it's smart marketing.

The two kinds of oil, although from the same plant, are quite different. Palm oil contains just about equal amounts of saturated and unsaturated fatty acids, whereas palm kernel oil is about 82 percent saturated. Neither contains any trans fatty acids because they have not been hydrogenated. (With all those saturated fatty acids, who needs saturation-producing hydrogenation?) Palm oil is sometimes subjected to "fractionation," meaning that its fatty acids are partially separated from one another to produce oils with different balances of saturated to unsaturated fatty acids. Without knowing which fractionated product you're getting in your margarine, you don't know its ratio of saturated to unsaturated fatty acids.

Most authorities therefore recommend that because of their high saturation levels -- 50 percent for palm oil, 82 percent for palm kernel oil -- it's a good idea to minimize your consumption of any oil from a palm tree, also known as a "tropical" oil.

LABELINGO: Perspicacious reader Michael F. Jacobson of Washington noticed that skim milk sold in the Whole Foods Market claims to be homogenized -- with the fat distributed uniformly throughout the liquid. He wonders how and why they would homogenize milk that contains no fat.

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th Street, NW, Washington, D.C., 20071 or to the e-mail address below.)

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W. W. Norton, hardcover, 2002). He can be reached at wolke@pitt.edu.