Related Free Slurpees come with a twist: They may cost you USA Today

Slurpees are free today at 7-Eleven. And, oddly enough, if consumers act like they have on previous Free Slurpee Days, the number of customers paying for Slurpees will soar today.

Last year on Free Slurpee Day, 7-Eleven slushed out 4.5 million free Slurpees—at the same time that Slurpee sales increased 38% that day, reports USA Today.

Logically, one would think that Slurpee sales would dramatically decrease on a day in which customers can get their frozen fix for free. But consumers aren’t entirely logical creatures. 7-Eleven says that once many shoppers get a taste of the free 11 oz. Slurpee, they just have to have more—even if they must pay for it. That’s apparently what a large number of consumers do.

Now, I can understand how a consumer swinging by a store to snag a freebie might buy something else while there. Perhaps you get the free Slurpee, and grab a Slim Jim or donut or fill up the car with gas while you’re at it. But paying for a product on the day it’s given out for free? This is weird territory. Seems sorta along the lines of the power of free supermarket samples, in which customers feel obligated or somehow justify buying stuff simply because they’ve been given something without paying for it.

A psychology professor offers some insight to USA Today:

“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.”

No one ever said consumers are entirely rational creatures—and no one knows this better than marketing professionals.

Brad Tuttle is a reporter at TIME. Find him on Twitter at @bradrtuttle. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.