Bite into a slice of raw onion, and the taste is sharp, hot and explosive. The vapor, both powerful and tantalizing, burns the throat and stings the eyes. Cook it, and the vegetable mellows, acquiring a taste similar to the sweetness of caramel.

The onion's versatility, flavor and low-calorie content make it an all-purpose food. Most chefs use it to add spice and texture to recipes. Braver cooks serve the vegetable as a main dish -- either stewed with tomatoes, stuffed with rice, or tossed with pasta.

Onions are plentiful and cheap -- costing about 33 cents per pound. Americans consume about 18 pounds of onions per person each year, according to the United Fresh Fruit and Vegetable Association, a trade group based in Alexandria. The amount is expected to increase as people substitute onions for other high-fat foods.

With only 55 calories in a cup of chopped, raw onion, trace amounts of fat and no cholesterol, nutritionists believe that Americans can't go wrong adding more of the vegetable to their diet. "You can comfortably increase the amount of onion called for in a recipe without significantly increasing the calories," said Colleen Pierre, a Baltimore-based nutrition consultant.

The vegetable is also a good source of potassium -- with 248 milligrams per cup -- and has a fair amount of vitamin C -- 13 mg per cup. One medium-sized apple, for example, contains about 8 mg of vitamin C.

Onions -- a member of the lily family -- have sparked the interest of scientists who are investigating what effect, if any, the vegetable may have on retarding cancer. Based on preliminary research, certain compounds in an onion may prevent a cell from either becoming cancerous or hinder the uncontrolled growth of cancer cells, according to Vicente Notario, associate professor of radiation medicine at Georgetown University School of Medicine.

Although the onion has enjoyed a medicinal reputation since the time of the Egyptians and is considered among the world's oldest cultivated vegetables, surprisingly little research has been done to identify the bulb's molecular structure. Scientists suspect it may actually have thousands of compounds, said Terrance Leighton, professor of biochemistry at the University of California at Berkeley.

Some compounds are more powerful than others. Squeamish chefs avoid slicing raw onions because of propanethial S-oxide -- an ingredient that creates a mild form of sulfuric acid when mixed with any water, such as teardrops, and causes people to weep.

To avoid the stinging vapors, tortured chefs have tried methods that range from wearing swimming goggles or a scuba mask to rubbing lemon juice on the knife before cutting. The National Onion Association, an industry group in Greeley, Colo., suggests storing an onion in the refrigerator first. Then cut off the top of the onion and peel the outside layers while leaving the root end on. Slicing the onion under cold, running water is another alternative.