There's no denying that we love free stuff. We wait for hours in the cold for free pancakes; we flock by the thousands for free donuts; and we even engage in violent riots (armed with BB guns and knives on sticks) for free smartphones.

But the truth is, companies love hosting "free days" even more than we love waiting in ridiculously long lines to get our hands on stuff we could have purchased for a measly few bucks on any other day of the year. That's because giveaways are almost always guaranteed to boost sales, rather than cost the companies money (like you might expect).

These "free days" essentially take advantage of what marketing professionals call the reciprocity principle, which argues that consumers tend to return the favor when companies throw them a bone (or a free donut). It's proven by science, too. A 2005 study published in the Journal of Consumer Psychology found that feeling obligated to spend more money "can occur despite the fact that we may never have requested the favor in the first place."

For instance, here's how the reciprocity principle applies to the annual 7-Eleven Free Slurpee Day — which is coming up on 7/11 (get it?): When consumers are gifted a small Slurpee, many quickly decide to also purchase a bigger size (or various other items). "You get a taste of it, and you choose to have more," Slurpee Senior Brand Director Laura Gordon told USA Today. In fact, Slurpee sales actually increased by 38 percent on Free Slurpee Day in 2011, despite the chain giving away 4.5 million frozen drinks.

As an added benefit, "free days" also expose both new and old customers to products they may not have tried otherwise. "Companies pay a lot of money to simply make people aware of their brand or products (e.g., the Alfa Romeo Super Bowl sponsorship), but these initiatives [free days] will do all the work, with a local audience, in one day," explains Christina Cartwright, who's been working in marketing for 13 years. "People normally become aware of them via PR coverage and word-of-mouth (which is low-cost), and the compelling reason to try the product is the fact that it's free."

Research has, in fact, proven that freebies increase word-of-mouth surrounding both products and brands. Case in point: A 2013 report in the Journal of Marketing found that product giveaways increase word-of-mouth by 20 percent, whereas coupons and rebates didn't add anything to this conversation.

Welp, sounds like more proof that we're all just being played by capitalism.

Even still, I'll take that complimentary Slurpee come Free Slurpee Day. And while I'm at it, I'll buy myself a Slim Jim... And Doritos... And gum... And a lighter... And lighter fluid... And charcoal... And a newspaper... And a Sports Illustrated... And lotto tickets... And bacon... And Triscuits... And dog food... And a taquito... And a frozen pizza... And some TUMS... And a banana...

Ian Lecklitner is a staff writer at MEL. He last wrote about why NASA just rocketed human sperm into space.