Why do we have different measuring cups for wet and dry ingredients? A cup of sugar is the same volume as a cup of milk, isn't it?

That depends on what your definition of is is.

A cup is indeed a cup throughout our land: eight U.S. fluid ounces, whether wet or dry. But now you may be wondering, if a fluid ounce is a measure of fluids, how is it that we use it also to measure dry solids? And what's the difference between an ounce of volume and an ounce of weight?

The confusion stems from our idiotic American system of measurements. Here's what we were supposed to have learned in school (pay attention now, and follow the bouncing ounce):

A U.S. fluid ounce is an amount of volume or bulk and is to be distinguished from a British fluid ounce, which is a different amount of volume, both of which are to be distinguished from an avoirdupois ounce, which is not an amount of volume at all but an amount of weight and is to be distinguished from a troy ounce, which is a different amount of weight and is not to be distinguished from an apothecary's ounce, which is exactly the same as a troy ounce except in February, which has 28. Got it?

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Now if that isn't an argument for the metric system, where weight is always in grams and volume is always in liters, I don't know what is.

Let's rephrase your question. Aren't eight good old American fluid ounces of milk the same amount of volume as eight good old American fluid ounces of sugar?

They certainly are. We'd really be in trouble if that weren't the case. But we still need a set of glass measurers for liquids and a separate set of metal measurers for solids.

Try to measure out a cup of sugar in a two-cup glass measurer and you'll have a tough time judging exactly when the sugar reaches the one-cup mark, because the sugar's surface isn't completely level. But even after you tap it on the counter to flatten it out and adjust it exactly to the mark, you won't have the amount of sugar that the recipe intended. That's because the recipe tester used a metal, one-cup "dry" measurer, filled flat to the brim. And believe it or not, that gives you a different amount of sugar than if you measured it out in a glass measurer.

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Try it. Measure out exactly one cup of sugar by slightly over-filling a one-cup metal measurer and scraping off the excess with a straight edge, such as the back of a large chef's knife. Now pour the sugar into a two-cup glass measurer and jiggle it until the sugar's surface is flat. Betcha it doesn't come fully up to the one-cup line.

Could that be due to inaccuracies in the measuring cups themselves? Not unless you're using a flea-market-special with lines that look as if they were hand-painted in kindergarten; reputable kitchenware manufacturers are pretty careful about the accuracy of their products. No, the answer lies in a fundamental difference between liquids and granulated solids such as sugar, salt and flour.

When you pour a liquid into a container it flows down into every crevice, leaving no spaces, not even microscopic ones. But a granulated solid can settle unpredictably, depending on the shape and size of the grains and of the container. Generally, when poured into a wide container, the grains get a chance to spread out more and fill in the spaces beneath them, so they settle down more compactly than if they were stacked up in a narrow container. And because they are settled more densely, they occupy less volume. The same weight of sugar will therefore actually occupy less volume in a wide container than in a narrow one.

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Go into the kitchen and look at your measuring cups. Dollars to doughnuts you'll find that at the same capacity level, the diameter of your glass measuring cup is substantially wider than the mouth of your metal one. Therefore, sugar and especially flour, which is notorious for its erratic settling, will occupy less volume in the glass measurer. If you use a glass measurer for your dry ingredients, you'll be adding more than the recipe intended.

To nail this down, I tested the opposite effect: I poured a leveled metal measuring cup of sugar into a tall, narrow measuring vessel--a chemist's graduated cylinder. As I expected, it filled the cylinder quite a bit higher than the eight-ounce (237-milliliter) mark.

Modern glass measuring utensils are, unfortunately, even wider than their predecessors, probably because people today want to heat milk or other liquids in them in their microwave ovens, and those liquids won't froth or boil over in a wide container. So today's liquid measurers are particularly poor for measuring dry ingredients. But there's a problem even when measuring liquids in them. In a wide container, a small error in the filling height can make a relatively large error in the volume. Those big, wide-mouthed glass measurers are therefore not as precise in use as the older, narrower ones. If you still have one of the oldies, cherish it.

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The ultimate answer to accuracy in the kitchen is quite simple, but we Americans just won't do it: Instead of measuring ingredients by volume, weigh them, as the professional chefs do. A pound of sugar or flour is always the same amount of stuff, no matter what kind of container you put it in or how it settles. You can even weigh liquids like water and milk if you like, but there's no need to because a fluid ounce weighs exactly one avoirdupois ounce, so there is a direct correspondence between both kinds of ounces.

That may sound like the lone virtue of our American measuring system, but it's a trap. The ounce-for-ounce equality is true only for water. A fluid ounce of anything else may weigh more or less (usually more) than an avoirdupois ounce, so they're not generally interchangeable.

And then there's the ounce, cup, pint, quart and gallon befuddlement. Quick: How many cups in a quart?

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See what I mean?

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking question to the Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or e-mail it to wolke@pop.pitt.edu. Individual responses cannot be made, but we will try to address your questions in this column.

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