The tray comes out of the oven with alternating rows of sugar cookies, one a perfect pale gold color and the other a leathery brown. Minutes later, two slices of foil-wrapped steak are pulled from boiling water and displayed; one is much more shriveled than the other.

Yet even though it's lunchtime, the results of this cooking demonstration for an audience of cooking school students won't satisfy any appetites, because the lesson, taught by visiting food scientist Shirley Corriher, has produced food fit only for a laboratory. Corriher is there to talk not about the way to cook foods, but rather why foods cook the way they do.

Although the very word "science" is enough to make many eyes glaze over and send their gaze drifting toward the nearest window, Corriher is so expressive that her experiments come alive.

Corriher's recent guest appearance as a lecturer, an annual event at L'Academie de Cuisine in Bethesda, coincided with publication of "The Curious Cook" (North Point Press, $19.95) by another noted food scientist, Harold McGee.

McGee's first book, "On Food and Cooking" (MacMillan, 1984), was a pioneer in making the science accessible even for nonprofessionals, and McGee and Corriher, friends as well as colleagues, are the best-known practitioners of a small but growing field of science.

Both of McGee's books are worth exploring by anyone interested in the "whys" of cooking; as for Corriher, whose own book on food science is to be published by William Morrow next year, she has to rely for now on acting out her information.

She illustrates her lessons with a hand-held board and a purple pen, making molecular activity look like child's play. Literally.

"Here are emulsifiers with their little tails sticking out," she says, drawing little squiggles. The gold whisk charm on her necklace swings as she moves across the floor, animated, smiling and breathless. "Remember our proteins tightening up?" she says, squeezing her arms together and grimacing.

She also gets into the act when she explains why the drumsticks of a chicken are never done by the time the rest of the bird is perfectly cooked.

"The dark meat is covered with collagen, and has to be heated to a high enough temperature to melt the collagen, and by that time the breast is overcooked," she says, and rubs her arms as though applying the collagen as a cosmetic.

Now it seems perfectly clear. Unfortunately, however, neither she nor McGee can recommend a solution other than either cooking the legs and breast separately or putting foil over the breast to slow the heating process.

But the two do have answers for just about everything else, such as why a cake sinks in the middle even when it seems to have cooked long enough and cooked through; apparently, it means the cake has too much or too little baking powder.

"There are an amazing number of ways for things to go wrong," says McGee, "but if you know what's the basic story, you can know where to start, rather than wondering what went wrong."

Understanding the reasons foods react the way they do is most helpful when following someone else's instructions. Even cookbook, magazine and newspaper recipes can be maddeningly vague.

"All of us have had the experience of following a recipe, we think exactly, and it doesn't work," says McGee. "We are assuming that the person who wrote the recipe knew what he or she was doing, and when we miss something or when a recipe is badly written, we don't know how to cope."

For instance, imagine making a chocolate sauce to pour over poached pears. The instructions in the recipe simply say to melt the chocolate over simmering water on low heat. But for some reason when the chocolate is almost completely melted, it suddenly begins to lose its gloss and harden.

Thinking that the chocolate sauce needs some liquid to smooth it out, the cook adds a tablespoon of cream from the refrigerator, and ... yikes! The entire mixture seems to almost climb out of the pan and form a hard ball of dull, tacky brown stuff. The chocolate has "seized."

What happened is that the particles of chocolate started to cling to the slightest molecules of moisture -- something that may have been present if the chocolate bars were too warm to begin with, causing moisture to form on the surface. That's what started the seizing, and when a little more liquid was added (the cream), the chocolate went wild.

Corriher calls this the "sugar bowl effect." "If you dip a wet spoon into a bowl of sugar, the sugar all clings to the spoon," she explains. "But if you put a lot of liquid into the bowl, the sugar dissolves." The chocolate sauce could have been saved instead of discarded if the cook simply stirred in a lot more cream (rather than a little).

But the cookbook never explains this.

The biggest problem in following a mass-publication recipe, "bar none," says Corriher, is the difference in flour around the country. The amount of protein in flour varies by region, but an "all-purpose" label can be applied to both low and high protein flours. To further complicate matters, degrees of humidity and temperature also affect the baking process.

"The term 'all-purpose' means nothing," says Corriher. "If Mrs. X from Montana is vacationing in Florida and buys a Junior League cookbook and goes home and tries to make the biscuits with her flour, it won't work." This is relevant information to anyone who has wasted several cups of flour and the better part of an afternoon trying to make wheaty loaves of bread or Southern-style flaky biscuits.

Although both McGee and Corriher know how to cook well, they necessarily spend most of their time making messes. "People say, 'Shirley makes the worst things,' " says Corriher with a laugh. McGee confesses to getting a similar reception at home, when he proves his theories in the family kitchen.

"My wife will come home and sort of shudder when she looks in the sink and sees the remains of the day's experiments there," he says. "But I am a good cook."

The Atlanta-based Corriher works as a consultant for restaurants and magazines as well as teaching food science to amateurs and professionals around the country, and got her start with a bachelor's degree in biochemistry.

McGee also thought he would become a scientist when he was an undergraduate, but ended up getting a master's degree in literature instead. It was only after helping his wife, a scientist, with her dissertation on the biochemistry of bean-seed development, that he began to research the science of food.

He discovered that people had lots of questions for him -- from someone concerned that aluminum pans cause Alzheimer's disease (he has concluded they do not), to someone having no luck at all making gelatin desserts.

"Each problem is like a little miniature mystery," he says.

For everyday cooks, all kinds of basic questions can be answered by an understanding of food science, once they realize that preparing a dish really involves applying principles of chemistry and botany, rather that just having a magical and elusive talent for cooking.

(The "bad" batch of cookies that Corriher produced for the students, by the way, turned brown because it was made with a very high protein flour; and the shriveled slice of beef was cooked in rapidly boiling water while the other was poached at a simmer.)

People who understand food science, says McGee, "are much more ready to cope with problems, and they are also much more interested and eager to experiment and try things a little differently."

"If you know the limiting factor in a dish," concurs Corriher, "you're free to go wild."