Free Slurpees come with a twist: They may cost you

The slurp's on them. But the goof may be on you.

Today 7-Eleven, the nation's largest convenience store chain, expects to give away 5 million 7.11-ounce Slurpees on the chain's unofficial birthday: 7/11. That's roughly 1,000 freebie Slurpees per store.

No coupons. No catches. No questions asked.

Those 7-Eleven folks sure must be dumb — as foxes. When they handed out 4.5 million Slurpee's last July 11, a funny thing happened: Slurpee sales for the day rocketed 38%, says Nancy Smith, vice president of marketing.

That's right: Folks bought more Slurpees, even though they could grab as many free tiny ones as they wanted. "You get a taste of it," says Slurpee senior brand director Laura Gordon, "and you choose to have more."

Perhaps. But many folks seem to be so enamored of the word "free" that they'll spend whatever it takes to cash in. In many cases, customers will spend more on gasoline just to get to 7-Eleven and wait in line for a free Slurpee than the estimated $1 retail value of the corn-syrup-laden drink that has virtually no nutritional value.

"Slurpee drinkers are some of the most loyal fans we have," Smith says. "They come here to have fun." And while they're in the store, Smith says, many buy other stuff, too.

What is it that drives consumers to chase after — even spend more to get — freebies? Denny's has seen record crowds when it's given away breakfasts. Ditto for free chicken wings at KFC and free doughnuts at Dunkin' Donuts. "Free is magic," says Barry Schwartz, professor of psychology at Swarthmore College. "If you offer something for free, people will gladly spend money to get it."

It gets sillier: People who go to Slurpee.com can download tiny hats and confetti to celebrate, photograph themselves, then post the picture on the Slurpee Facebook page for prizes.

Schwartz isn't buying — so to speak. "Economists are wrong about almost everything," he says. "But they're right about one thing: There is no free lunch — or in this case, free Slurpee."