What makes Sherry a separate category of wine? Is it the grape, the region, the method of production?

It's all three.

I must first confess that I'm far from an expert on wine. Frankly, I find the 5,000 varieties (literally) of wine grapes plus the almost infinite number of combinations of varieties, regions and vintages to be absolutely mind-boggling, and I long ago gave up trying to learn about them. I would as soon try to learn the name, birthplace and age of every person in North America, together with precisely what characteristics I like or dislike about each.

Even more frankly, I am intimidated by the ravings (often in both senses of the word) of my friends about the subtleties of whatever wine happens to be in the spotlight. I deem myself fortunate, however, that I do have such knowledgeable friends, who see to it that I get to drink excellent wines without my having to ferret them out.

Before you despair of having asked your question of the wrong guy, let me say that I do know something about Sherry, having recently visited the one place in the world where it is produced: in and around the town of Jerez de la Frontera, a couple of hours' drive south of Seville in Spain's province of Cadiz. There, I was figuratively and almost literally immersed in Sherry for five days, as I toured the headquarters of Williams & Humbert, producers of Dry Sack, Pando, Canasta Cream and many other Sherries and brandies.

Why the non-Spanish names Williams and Humbert? And whence the English word "Sherry"? Several of the Sherry companies in Jerez were founded in the 19th century by British entrepreneurs for the purpose of exporting Sherry to England, where the dry Sherries have always been favored aperitifs and the sweet ones favored as dessert wines. The word "Sherry" evolved into English from the name Jerez (HER-eth), but throughout the Spanish-speaking world Sherry is still known as vino de Jerez.

The identity of this Sherry is tightly controlled by a regulatory council. To earn the Denomination of Origin "Jerez-Xeres-Sherry," the grapes must be Palomino, or less commonly Pedro Ximenez and Muscatel, and they must be grown within the triangle formed by the towns of Jerez de la Frontera, Sanlucar de Barrameda and El Puerto de Santa Maria. This small region has a unique microclimate influenced by the Atlantic Ocean, the Guadalquivir and Guadalete rivers and the moist, warm winds from nearby North Africa. Probably the most influential factor in developing the character of the grapes is the area's chalky, almost-white albariza soil, which has an unusual capacity for absorbing and retaining air and water.

Of course, many great wines come from exceptional microclimates and soils. But what sets Sherries apart from all others is the unique process by which they are blended and aged.

After the grapes are pressed, the "must," as the pressings are called, is put into huge stainless-steel tanks to ferment for 40 to 50 days at a controlled temperature, achieving an alcoholic content of 11 percent to 13 percent. The young wine then goes into 130-gallon American white oak casks (butts), where the aging begins.

Then comes decision time, when each wine is classified as being suited for transformation into either a dry Fino or a sweeter Oloroso, the two broad categories of Sherry. Finos include Manzanillas and Amontillados, while Olorosos include Cortados and various blends of the very sweet Pedro Ximenez grape. Wines destined to be Finos are fortified to 15 percent alcohol, Olorosos to about 17 percent. The reason for the difference is that the flor (literally, flower), a layer of local, naturally occurring yeasts that forms on the surface, cannot survive at higher alcohol concentrations than 15 percent, and all Finos must serve out their aging time under a layer of flor to develop their characteristic lightness and flavor. Olorosos are aged without a layer of flor, which allows air to oxidize them to a darker color, fuller body and stronger nose. (Oloroso means fragrant.) Amontillados begin their aging under flor and finish after a flor-killing fortification to 17 percent alcohol.

During the aging, an intricate blending process unique to Sherries is carried out. Called soleras y criaderas, it consists of running the wine through a series of butts containing wines of increasing ages. From the oldest, called the solera, one-third of the wine is drawn off for bottling. It is replenished from the next oldest, which is replenished in turn from the next oldest, and so on, until the youngest wine, a criadera, is "refreshed" with new wine. The complete cycle takes years, with several months of aging time between successive solera bottlings. By this method, the young wines gradually take on the characteristics of the older ones, leading to a completely uniform product.

Inside the Williams & Humbert bodega, we gaped at thousands of butts piled to the high ceiling, each one waiting to have some of its contents moved to the next in the sequence.

Then, on to the tasting room, where we sampled, among others, a clean, bone-dry Dry Sack Fino with a broad-sword palate and an aroma reminiscent of almonds; a Dry Sack Medium Dry that was full on the palate, harmonious and mellow; a smooth, velvety Canasta with a hint of nuts and a touch of burnt sugar; a 30-year-old Amontillado that was robust and full, with a long and smooth aftertaste; and a 20-year-old Solera Especial Palo Cortado, strongly aromatic with a woody bouquet that blended perfectly with its full nutty palate and finishing with a long and intense aftertaste.

Okay, so that paragraph doesn't sound like me. I copied these descriptions from Williams & Humbert's literature. As I said at the beginning, I'm far from an expert.

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 Barber: More Scientific Answers to Everyday Questions" (Dell Publications, $12.95). His next book, "What Einstein Told His Cook," will be published by W.W. Norton in next spring. Send your kitchen questions to wolke@pitt.edu.