Whether it’s mayo, ketchup, or gravy, French fries are usually served with something to dip them in. But diners have been known to go rogue and make their own dipping sauces out of whatever they can find. Such cases of culinary ingenuity led to the marriage of milkshakes and fries, one of oddest—and somehow, most delicious—pairings in the fast food world. It seems to go against logic, but the combination’s appeal is backed by the science of taste.
Though they’re listed on different sections of the menu, milkshakes and French fries make more sense together than you might assume. Many famous food pairings—like chicken and waffles, maple and bacon, and chocolate and pretzels—combine salty and sweet. This pairing stimulates taste buds that aren’t active in the presence of sugar alone. According to a 2011 study in the Proceedings of the National Academy of Science, sweet and salty foods activate sugar receptors known as SGLT1 on the tongue’s sweet taste cells. Thanks to these receptors, salt enhances sweetness rather than clashing with it or canceling it out.
Fries and milkshakes also have the hot-and-cold dynamic on their side. Like distinct flavors layered into a dish, contrasting temperatures fire up different parts of our brain, leading to a more interesting—and more pleasant—eating experience. As Thrillist explains, this is why ice cream is often paired with warm foods, like apple pie, hot fudge, and fresh-baked brownies.
The physiology of taste is just one factor that determines the foods we love. Culture and peer pressure also play a huge role, which is why some flavors embraced in certain parts of the world are reviled in others. Dipping French fries in milkshakes is something many people start doing as children. Because the items aren’t listed together on the menu, kids learn to put them together from people in their social circles, like a parent, older sibling, or the captain of their soccer team. This encourages them to open their minds to the otherwise unusual matchup.
Though they didn’t come up with it themselves, many businesses have embraced the off-menu combo. McDonald’s and Wendy’s have both promoted dipping fries in milkshakes (or Frostys, in Wendy’s case) on social media. Some restaurants serve upscale takes on the classic that include fries as a milkshake topping—though for people who grew up with the pairing, the DIY version is hard to beat.