They say that there is no such thing as a free lunch, but "they" must not be shopping at the right grocery stores. Otherwise, they would know that it is possible to eat lunch for nothing in this town (and in the 'burbs) if you don't mind an eclectic menu -- say, chicken pot stickers followed by salmon burgers, with a dried mango chaser. You also can't be shy about going back for seconds, because the portions are small -- sample-size, in fact.

Food demo-ing or in-store sampling is a sales technique that says, "Give customers a taste, and they will buy." Most supermarkets have the occasional sampling table or two, but a few area grocery stores, such as Costco and Whole Foods Markets, go all out. These two chains could hardly be more different, but both are believers in the benefits of offering freebies.

Club Demonstration Services Inc., the company that handles the food demonstrations for Costco, tracks product sales after a demo. Fully 89 percent of the time, sales increase, according to Tammie Allen, regional manager.

"The sale may not happen that day, but they come back for it," she says.

And customers like the practice:

The trade journal Supermarket Retail Marketing recently reported that 70 percent of respondents in a 1,000-person survey said they would shop at a store if they knew it offered product samples. Of that group, 86 percent also said they would be more likely to buy a new brand if they could try it first at the store.

On a recent busy Saturday at the Leesburg Costco, a dozen food demos were in full swing. Patricia Eala, who has been demo-ing for 10 years, had a setup near the produce section. "Golden pineapple" was her sample du jour.

"I'm a talker," said Eala cheerfully. Being a people person is one of the main qualifications for the part-time job of food demonstrator. As she chatted, Eala was in constant motion, stabbing generous pieces of pineapple with toothpicks, sticking them in paper holders and setting them out for the shoppers. She wore gloves, a plastic hair covering and an apron and has been "food safety certified" by Costco. Her supervisors are certified on food safety by the state of Virginia.

People parked and double-parked their oversize carts to snag a taste of Eala's pineapple. Some lingered to ask questions: "How do you tell it's ripe?"

"What's the carb count?"

Others studied the nutritional information provided for the product at every demo station.

Eala kept slicing. "My boys are gone. And the kids are so precious. Future buyers, you know," she said.

One future buyer was hovering nearby, working up his nerve to come close enough to take a sample. Anthony Zorb, 7, shyly reached for a piece of pineapple.

"Honey," Eala said, "you have to be sure that's okay with your mommy. That you don't have any allergies." Food demonstrators are not allowed to serve children without their parents' permission.

Just then, Anthony's mother, Melanie, rolled up, her cart filled with groceries and two more children. She gave Anthony the nod. She tried a piece, too. The Zorbs did not commit to buying just then, but a remarkable number of the passing carts did have the big fruit aboard.

Meanwhile, Odessa Scott, stationed at the end of an aisle in frozen foods that day, was pushing pork sausage patties. Barbara Pirrone of Delaplane was nibbling. "I came to buy flowers," she said. "I didn't come to buy food." Brave words, but a definite wobble in her resolve was detectable. Sampling feeds the impulse buy.

For example, only two boxes of Margaritaville Calypso Coconut Shrimp ($11.69) sold at the Leesburg Costco in the week before the product was demo-ed recently. As shopper Warren Grossman of Purcellville pointed out, "The quantity is so large" -- Costco being a warehouse operation -- "and your commitment is so big" that shoppers might have been reluctant to take a chance.

The modest goal of the shrimp sampling, Allen says, was to sell 10 boxes, but on the day of the demo, 32 went through the checkout. The food company pays about $125 for a 6 1/2-hour demonstration. Demonstrators at Costco earn on average $9.50 an hour, no commissions. But money seems to be of secondary importance to some of them. "I've been in food service all my life," said Erika Floro, who was handing out granola near the bakery section. "I like to work with food, and I meet so many different people. I don't like to sit home and worry about my health." Floro is 87.

At the Whole Foods Market in Arlington, some grocery carts are doll-size compared with the Incredible Hulk carriages at Costco. But the shoppers still sample, then buy on the spur of the moment.

"Our shoppers aren't familiar with the products and the brands," says Sarah Kenney, mid-Atlantic marketing director for Whole Foods, explaining why the upscale grocery chain does so much sampling.

"We expand their horizons. We've turned people on to a lot of things."

Unlike Costco, Whole Foods doesn't track sales of the products it demos. Instead, it sees sampling as a manifestation of its overall ethos of food.

"Our approach is to share the food," Kenney explains. "REI has a climbing wall, and a bookstore lets you look at magazines. Sampling is part of our culture."

Kenney says that every Whole Foods outlet has its share of "professional samplers" -- people who come in regularly to graze on the giveaways. No one minds. (No one minds at Costco, either).

Whole Foods keeps its demonstrations in-house or has food vendors come in to demonstrate their own products.

Scott Sechler, owner of Bell & Evans ("Our chickens eat better so you eat better") periodically comes down from Fredericksburg, Pa., to do food demos in Whole Foods's Washington area stores. His "passion for chicken knows no bounds," Kenney says. "He will talk about chicken until the cows come home."

Whole Foods does lots of what it calls "passive sampling," too, meaning it has unmanned, help-yourself stations scattered throughout its stores. A store might have anywhere from 10 to 40 passive sampling stations set up on a given day.

The market avoids putting out anything that might provoke an allergic reaction, and it generally supplies product labels at the sampling stations so that people with special diets can exercise due diligence.

"But can I tell people that a person put the used toothpick in the used toothpick thing?" Kenney says. "I can't.

"Do I believe for a minute that anything is unsafe? I don't.

"But if people have concerns with germs, they don't need to do it."

So if food sampling is such a sure-fire sales technique, why don't all supermarkets do it big time, all the time?

Caroline Cotten Nakken, chief executive of Mass Connections, which handles sampling for many grocery chains including Giant, Food Lion, Shoppers Food Warehouse and Harris Teeter, says that retailers have a long list of items they would like to have manufacturers spend money on -- in addition to sampling.

But, "in fact the mass merchandisers and retailers see the value in heavily supporting in-store sampling and retail entertainment, and therefore have chosen to move the funds accordingly. We have seen an increase as much as 64 percent in in-store sampling since last year, " she says.

Jamie Miller, public affairs manager for Giant Food LLC, says the Dutch-owned chain expects to expand its food demonstration scheduling in the future but doesn't plan anything on the scale of a Costco or Whole Foods.

"They haven't got on the bandwagon yet," says Allen that Saturday in Leesburg, as she surveyed all the pineapple rolling past in grocery carts.

"If they saw the value of it, they would do it."

M.J. McAteer is The Post's letters editor and a freelance writer.