No, it's not the salt.

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There are matchstick fries and steak fries; crinkle-cut fries and waffle fries; cheese fries and sweet potato fries. But among all the rest, McDonald’s French fries are an old standby. There’s something about the flavor that we just. Can’t. Stop. Eating.

No, it’s not just in your head that fast-food fries are one of the most addictive foods. One secret ingredient keeps you coming back for more.

To make McDonald’s fries, fresh potatoes are washed, peeled, cut, and blanched in a factory, according to a video from the company. The plant also adds chemicals to keep the potatoes a uniform light yellow color (but no, that’s not behind their addictive flavor). After that, the cut potatoes are fried for less than a minute before getting frozen and sent to McDonald’s locations. At the restaurant, those frozen strips are cooked in oil and salt before getting landing on your tray in that red box.

That all sounds standard enough, so why are they so exceptionally scrumptious?

It all started in the 1950s, when the shortening company that gave McDonald’s vegetable oil couldn’t afford the equipment needed to hydrogenate the oil, which would extend its shelf life. (Learn how to tell if you eat too many preservatives.) So the supplier gave a blend of oil and beef fat instead, according to NPR.

Over time, McDonald’s and other fast-food joints made the beef fat part of their signature fry flavors. But in the 1980s, fast-food restaurants took the ingredient out when health advocates criticized how much “bad” saturated fat it added. (Find out if you’ve been eating too many bad fats.)

McDonald’s wanted to keep its signature beefy flavor but without the beef fat itself, so it came up with a solution. Now, the fast-food chain adds “natural beef flavor” to its vegetable oil to give its fries their irresistibly meaty (though not-so-vegan-friendly) taste. Next, check out these 75 mind-blowing McDonald’s facts.