Q. Hope you can set us straight on the following question. There are a lot of bragging rights involved.

My daughter and her friends think I'm nuts because I claim that it doesn't matter whether you eat a pound of high-fat chocolate or a pound of low-fat anything, you will gain a pound. They think that, if you eat a pound of chocolate, you can gain more than a pound. I can't understand where the extra weight would come from.

Jake Sullivan

McLean

A. We referred this one to The Post's perpetually trim medical writer Don Colburn. His report:

It's a good question, even if the bragging rights for both sides turn out to be pretty low-cal.

Dad's basic argument -- that you can't gain more than a pound from eating a pound of anything -- has at least an ounce of truth to it. But it's not true that all pounds are equal when it comes to eating.

What matters for weight gain is how much energy content, measured in calories, a food provides when it is absorbed by the body (and how much energy the eater burns off through normal metabolism and physical activity). People gain weight if they take in more food energy than they burn off. Every extra 3,500 calories consumed -- whether from chocolate or chop suey or chimichangas -- adds a pound of body fat.

A food's energy content per pound, known as caloric density, is the key measure here. And caloric density varies widely from food to food, says Ronald Krauss, head of molecular medicine at the Lawrence Berkeley National Laboratory and former chairman of the American Heart Association's committee on nutrition.

A 1.5-ounce Hershey milk chocolate bar has 232 calories. A pound of chocolate -- about 11 Hershey bars -- would pack nearly 2,500 calories. By contrast, a pound of raw carrots provides only about 185 calories. A pound of plain, skim-milk yogurt has 153 calories.

A food's total weight is not a good measure of its caloric punch. Low-calorie carrots weigh a lot, but 88 percent of their weight is water.

As veteran dieters know, it gets more complicated. Individual metabolic rates differ. And not all calories are identical in their importance to health. People with diabetes, for example, must pay special attention to calories from sugar. People with heart disease may worry more about calories from saturated fat.

Even dad's point about the impossibility of gaining more than a pound by eating a pound of food, while literally true, may be more complicated than it seems, Krauss points out. Eating something sweet, he notes, "leads some people, at least, to want more. And one pound leads to another."

Why are grass and most other plants green? I know that it is because they contain chlorophyll, which is green. But why is chlorophyll green? Why not blue or red? If the purpose of the stuff is to transform sunlight -- from our yellow and orange sun -- wouldn't it be more efficient for it to be to be yellow or orange in order to absorb the sun's energy more efficiently?

Kevin Wolf

Arlington

At last, a question so good that it actually answers itself!

Chlorophyll is green precisely because of the sunshine it absorbs. The molecule is built to suck up blue and red light -- the short and long wavelengths at both ends of the visible spectrum.

Those happen to be exactly the wavelengths chlorophyll needs to do its miraculous job of using light energy to disassemble molecules of carbon dioxide and water and then to reshuffle their atoms into carbohydrates.

But photosynthesis doesn't use the middle wavelengths of the spectrum, so chlorophyll doesn't absorb green light at all. Instead, it is transmitted to your eye.

But even mighty chlorophyll must fade. And when it does -- typically in the fall, when chlorophyll decays in most plants -- it reveals the intense autumnal hues previously masked by chlorophyll's powerful absorption.

Lurking within your average leaf is a plentiful supply of carotenes (the molecule that makes carrots look orange by absorbing blue and indigo light), anthocyanins and flavonols. These are naturally yellow, orange and red. And although they can't make sugars, they can do something that chlorophyll can't: generate millions of tourist bucks every fall.

I have wine glasses that are "glass" and wine glasses that are "crystal." They all look like clear glass. What makes them different?

Claudia Kilmer

Manassas

Well, frankly, less than you'd think. The principal ingredient of most everything we call glass is silicon dioxide -- ordinary old sand -- which you may also know under its hifalutin alias of quartz.

Sand alone makes a passable sort of glass. But it has a melting point around 3,100 degrees F, an inconvenient temperature for inflammable mortals. So over the centuries, glassmakers have found additives that lower the melting point. The two most common are soda and lime. Your basic household water glass is made from this recipe.

The familiar kitchen and laboratory staple called Pyrex glass, however, is a bit different. Common glass has a nasty propensity to expand or contract a lot when heated or cooled; change its temperature too hastily, and it shatters. But a mix doped with a little boron oxide results in a glass with a much lower thermal expansion rate -- a trait much prized by cooks and the finicky flask-and-beaker set.

"Crystal" glass is produced by yet another formula. But first, a cruel truth: No matter how much you pay for it, it's almost certainly not a crystal. That term refers to materials in which the constituent atoms and molecules arrange themselves into orderly geometric patterns, which usually happens when molten things cool slowly.

Commercial glass, which is cooled quickly, is not crystalline; it is amorphous. That is, its constituent molecules are connected in random arrays with no regular spatial patterns, much like the arrangement of molecules in a liquid. (Some centuries-old windows have "run" over time, becoming thicker at the bottom as the glass oozes downward.)

Amorphous glass is easy to dope with compounds that influence color or other optical properties. One of those is lead, added in the form of lead oxide, which dramatically increases the refractive index of the glass. That is, it bends light more, which enhances its sparkle. Lead also adds to the weight (about one-fourth of the weight of good crystal is lead) and, of course, to its price.

Why does a wet finger stick to icy surfaces? If you wash your hands and touch something in the freezer, it can be difficult to detach yourself without leaving skin behind.

Dyanne Tsai

Gaithersburg

Get an ice cube, grab one end with each hand and try to pull the thing in half. You'll immediately notice one of the big differences between liquids and solids. The molecules of solids -- ice in this case -- are much more tightly bound to one another.

So if your wet finger freezes to the object in the freezer compartment, pulling it off is no easier than tearing an ice cube into pieces and will probably result in an involuntary skinectomy.

Why doesn't the ice just pop off your finger? Because it's wedged into your skin. The reason lies in the nifty way water molecules attach themselves to one another. Each has two atoms of hydrogen and one of oxygen. But the hydrogen atoms both sit on one side of the oxygen atom, sort of like Mickey Mouse ears. That makes water molecules electrically polar: The hydrogen end is slightly positive, and the bare oxygen end slightly negative.

In the liquid state, the oxygen side of each molecule typically binds itself to the hydrogen side of one or two other neighbors with an electrostatic tug called a hydrogen bond. The strength of those bonds accounts for water's relatively high boiling point.

As water freezes, there is less thermal motion to wiggle the atoms. Now each oxygen atom forms as many as four hydrogen bonds with adjacent molecules, locking it into a rigid crystalline structure in which four hydrogen atoms form a pyramid-like tetrahedral shape around each oxygen atom.

That arrangement contains a lot more empty space than its liquid counterpart. So water expands when it freezes. If it has worked its way into tiny crevices of skin, it anchors itself by swelling as it turns to ice. And you're stuck.

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