If you’ve ever walked up to a McDonald’s counter with the intention of ordering a normal-sized Coke and walked away with a full 32 ounces of fizzy liquid, you’re definitely not alone. Since all McDonald’s soft drinks are $1, it’s hard to pass up the opportunity to upsize your small to a large.

It seems strange that a business would be willing to sell such drastically different amounts for the exact same price, but it’s actually a savvy business tactic. McDonald’s introduced the $1-fits-all promotion for soft drinks back in 2017 as a way to compete with Wendy’s, Burger King, and other fast food chains that had been slowly stealing its customers. According to Reader’s Digest, it worked—sales increased by 4 percent, and the deal became a mainstay on the dollar menu.

Basically, discounted drinks generate revenue by getting you in the door, after which there’s a good chance you’ll end up purchasing something more expensive from the menu, especially with the tantalizing scents of fresh French fries and sizzling Big Macs wafting right to your brain’s pleasure center. And since production costs for soft drinks are so low, McDonald’s can afford to offer them for especially low prices.

But soda isn't the only item offered at a deep discount—McDonald’s has a whole Dollar Menu with food and drinks priced at $1, $2, and $3, which they know they can count on customers to use as a springboard for bigger, better, more costly orders.

[h/t Reader’s Digest]