Ripe wine grapes are sweet. If they weren't, there wouldn't be anything for the yeast to turn into alcohol for your drinking pleasure. Different grape varieties ripen at different rates, and one of the winemaker's most crucial decisions is when to pick, because after picking, no more sugar is formed. As grapes are left to hang on the vines far into the autumn—and if the weather gods are friendly—the grapes can become so sugar engorged that there's plenty to turn into alcohol and still have some left over. The result: a sweet wine, or in the disrespectful parlance of the trade, a "sticky." Not sweet enough for some tastes, however. Over the centuries, various tricks have been developed to concentrate the grapes' sweetness even more. The best such wines are among the rarest and most expensive beverages on earth—the Sauternes of France, the Tokay of Hungary, the Trockenbeerenauslesen of Germany. In cold climates, some winemakers let the thermometer do their work for them, leaving fruit to hang until a cold snap freezes it near solid. But sugar is a natural antifreeze, so the little liquid left unfrozen is supercharged with sugar. Properly handled, such juice can become the basis for "ice wine"—for obvious reasons, something of a Canadian specialty. But the most revered sweet wines come about more subtly: through a process of creative decay under the effects of a microorganism that grows naturally on grape skins. Under the right conditions, this mold (Botrytis cinerea) puts tendrils down through the skin, sucking water out of grapes to keep itself alive, concentrating the remaining juice in the process. It was a wise or reckless winemaker who first tried to create a potable beverage from such fruit, because it more closely resembles something long forgotten at the back of the crisper than the source of $500-a-bottle nectar. All sweet wines—white wines—taste a good deal alike, because pure sugar is their principal flavor component. What makes the great ones precious to connoisseurs is that the sweetness is balanced by acidity. That's why great sweet wine is rare—because as grapes ripen, acidity falls as rapidly as sugar accumulates. But clever winemakers can produce luscious results from many different grape varieties. That's what Erik Liedholm, sommelier of Seastar Restaurant in Bellevue, hopes to demonstrate at his seminar on Taste Washington's Education Day, through tasting dry and sweet versions of the same grape side by side. Taste Washington Education Day: Sat., April 9, at Bell Harbor International Conference Center, 2211 Alaskan Way, Pier 66. Erik Liedholm's seminar in "White Wines: Dry and Sweet" takes place at 9:30 a.m. Tickets $45; for a full schedule and information on how to purchase, go to www.tastewashington.org.