Q: What is an itch? How is it formed? How does scratching it sooth it?

Jennifer Hursh (age 10)

Centreville

A: Itch falls into the category of things that are irritating without being outright painful, sort of like the Fox Network. It is definitely akin to pain. It travels on similar nerve routes and is carried by pain-type nerves, the so-called C fibers that lack an outer sheath of insulation, like an electrical wire without its plastic coating.

C fibers make up a substantial fraction of the million or so nerve endings that send signals from your skin to the central nervous system.

But an itch is not an ouch. Unlike pain, itching does not arise in deep tissues. Remove the top few layers of skin, and you've removed the ability to itch.

High-tech scans show that itching affects somewhat different parts of your brain than does pain from the same area and that itching seems to travel on its own specific nerve tracks. Those fibers are particularly sensitive to the effects of histamine, a substance released by the body in response to damage or invasion of one sort or another.

The itch signal is easily overridden by pain or pain-like nerve impulses. That's probably why we share a "scratch reflex" with many other vertebrates. A good scratch is close enough to pain so that it blocks the itch sensation temporarily while also possibly scraping the offending material off the skin.

Cold reduces sensitivity to itch. So does naxolone, the same drug that blocks the effect of opiates on heroin addicts. The opposite also is true. Inject the brains of rats or monkeys with morphine, and they start scratching their faces.

What causes an itch in the first place? A better question might be: What doesn't? Most itching derives from simple causes such as dry air or overwashing. Both conditions allow the skin to crack and admit foreign substances to which you may be sensitive.

Soaps, detergents, fiberglass, poison ivy oil, miscellaneous animal hair and other, um, pet byproducts can all get under your skin.

Then there's scabies -- mites that burrow into your skin -- and a host of skin conditions including psoriasis (from psora, Greek for itch), rosacea and eczema (causes unknown), erythema (produced by a virus). Also, there are ringworm, jock itch and athlete's foot, all brought on by basically the same fungus.

Don't have those, but you're still scratching? The cause may be numerous diseases, including chickenpox, diabetes, thyroid complaints, liver and kidney disease, iron deficiency, certain cancers and blood disorders.

Lots of drug reactions and allergies entail itching, as do several kinds of mental illness and, in rare cases, pregnancy. Put it all together, and it seems amazing that we don't spend most of our waking hours scraping away at our pelts.

Q: All other factors being equal, which will come to a boil first: a pot of cold tap water or a pot of hot tap water? My wife thinks that it's the hot water because it's already closer to boiling temperature and the head start should allow it to boil faster. But I seem to remember something from high school physics about cold water boiling faster because it starts off denser and thus the heat from the burner gets the water churning faster. Who's correct?

Bobbie Kerns

Spotsylvania, Va.

A: As is so astonishingly often the case in modern life, the wife is right. True, you salvage some dignity by having recalled, correctly, (1) that cold water is usually more dense than warm water, and that (2) temperature and density have something to do with churning.

It's more commonly called convection -- the vertical motion caused by density differences in fluids, which accounts for the motion in a teapot and a thunderstorm.

Nonetheless, the hot pot boils first. But not much sooner. Surprisingly little of the total heat energy expended in boiling a quantity of water is used to raise the temperature to the boiling point. The vast majority goes into changing the water from a liquid to a gaseous state, without raising the temperature at all.

In fact, if you add heat at the same constant rate to a container of water at 328F and time the process until every drop has boiled away, you'll find that the contents hit 2128F in about one-fifth of the total elapsed time.

The other four-fifths are required to turn the liquid into steam. That's why it seems that a watched pot doesn't boil.

Q: Why is it that burned toast and a burned pork chop taste the same? They seem to have little in common.

Ann Bietsch

Shippensburg, Pa.

A: If they taste exactly the same, perhaps you ought to change restaurants. But the fact that they taste somewhat similar shouldn't be terribly surprising. Both the average wheat plant (the source of the bread) and the basic deceased pig (unwilling donor of the chop) are what we call carbon-based life forms.

About 19 percent of the atoms in the human body -- which is embarrassingly close to a pig's in basic physiology -- are carbon, and 63 percent are oxygen. The composition in alfalfa is pretty close -- 11 percent carbon and 78 percent oxygen.

So when you incinerate either, you get a fairly comparable result, namely, a lot of carbon dioxide, some yucky residues and a lump of glorified charcoal.

What you're tasting is the carbon lump, together with whatever impurities survived combustion. And that may not be healthy, warns our chemistry adviser, Joe Schwarcz, whose latest article begins on Page H1.

Charring meat tends to produce polycyclic aromatic hydrocarbons and heterocyclic aromatic amines. Never mind the names. Both are known to cause cancer in lab animals.

Moreover, charcoal-broiling your chop (or, for that matter, your toast) can emit a related compound called benzopyrene, also found abundantly in cigarette smoke. It's routinely used to induce cancer and cell mutations in experiments.

How about a nice bowl of steamed broccoli?

Q: I am nearsighted, which means that I can see close-up objects fine but not objects far away. I would expect that, when I'm sitting in my car and look into the rear-view mirror, I should be able to see the reflected image perfectly since the mirror is only a couple of feet away. Instead, the image appears just as blurry as if I was looking at the far-off object directly. How come?

Don Evans

District

A: Congratulations. You have verified that life not only is unfair but also is sometimes painfully illogical. Presumably, you can focus sharply on the rear-view mirror housing. So it seems to make perfectly good sense that you also should be able to focus just as easily on the image in the mirror's reflective plane.

And so you could if the image had been painted on the mirror or if you'd pasted a photo there. But the reflected image doesn't really exist on the mirror plane in the same sense as paint or photos.

Instead, the mirror provides a way for light beams diverging from a source to be deflected into your eyes. To focus properly, you still must trace those beams back to the point where they originated.

We asked our trusty physics adviser, Professor Drew Baden of the University of Maryland physics department, to explain. His response:

"In the field of optics, image is everything. If you look at the image of some object in a flat mirror, the position of the image seems to be directly behind the mirror. For instance, if you look at your own face in a mirror and you're standing four feet in front of the mirror, then you'll see the image appear as if it were four feet behind the mirror -- a total of eight feet away from you.

"So if you need glasses to see things eight feet away, you'll also need them to see your own reflection. That's the thing about images -- as far as the eye and brain are concerned, they're real!"

Readers who have trouble focusing on things that are close, a typical result of aging, can test the same principle.

Take a hand mirror and make an "X" on it with wax pencil or whatever. Then hold the mirror in front of your face and bring it close enough so that you can't focus on the "X."

To your astonishment, you will be able to focus easily on the reflected image, even though the mirror surface itself is out of focus. Reflect on that.

Q: I took two equal-sized stainless steel bowls, one heaped to the brim with wheat flour and the second similarly filled with water. I gradually mixed both in another container to make dough. When it was ready, I expected the dough to fill both of the original steel bowls. Surprisingly, it was nearly accommodated in one. How come the volume of both ingredients was reduced to almost half their combined original volume?

Raj Tilak

Burtonsville

A: We assigned the fearless Horizon kitchen test team -- Samantha and Maria Suplee -- to check the facts in this matter. They determined that one full cup of water and an equal volume of all-purpose flour combine to produce about 1.3 cups of batter.

The principal reason for the shrinkage, of course, is that water occupies the fairly considerable open space between granules of dry flour. Those bits range in size from about twice the width of a hair for hard pasta flour to one-tenth that thickness for spongy cake flour. Water molecules are tens of thousands of times smaller. "They squeeze in between, like sand in a pile of golf balls," says Shirley O. Corriher, biochemist-turned-author of Cookwise: The Hows and Whys of Successful Cooking.

Beyond that, there are a couple of other processes at work, and we will provide them on a knead-to-know basis. They are taken from one of the few dozen utterly indispensable popular-science books in modern life. That's On Food and Cooking: The Science and Lore of the Kitchen by Harold McGee. If you haven't got it, you don't get it.

General-purpose flour contains about 11 percent protein and 76 percent carbohydrates. Both absorb water. Some of the proteins -- notably those that form gluten and give bread its elastic, chewy texture -- are loopy, coiled molecules that absorb about twice their weight in water, tucking the liquid into available spaces in the coils.

In addition, the protein strands stretch as they become wet and line up more or less in parallel -- a much more compact arrangement.

It's a bit like "shaking a box full of randomly oriented pencils," McGee writes, until they tend to line up in orderly stacks of less volume. This makes the dough stiffer and more elastic, an effect also encouraged by kneading. Thereafter, the dough rises to the occasion.

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