What began as a hot dog stand in New York’s Madison Square Park (yes, a hot dog stand) has exploded into a burger empire with locations across the globe. Shake Shack, the brainchild of restaurateur Danny Meyer, has discovered a sweet spot with fast-food-weary customers, and spawned numerous imitators along the way. Here, we take a look at the company’s beginnings, where it’s headed, and whether peanut butter ShackBurgers are really a thing.

1. IT STARTED AS PART OF AN ART INSTALLATION.

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Back in summer 2001, an art show called “I  Taxi” took over Madison Square Park. In addition to all sorts of taxi-themed displays, there was a hot dog stand that quickly became a hit for its friendly service and Chicago-style dogs. Little did patrons know, it was actually run by restaurateur Danny Meyer (who headed up the Madison Square Park Conservancy) and staffed by off-season coat-check workers from his upscale restaurants. The operation lost money over the three summers it was in business, but Meyer was encouraged by the turnout. So he asked the Parks Department for a full-time business permit, pledging to donate some of the earnings to the park’s development, and they obliged. In 2004, Shake Shack opened, and notoriously long lines ensued.

2. IT GOT ITS NAME FROM THE MOVIE GREASE...

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Meyer told Fortune he must have come up with the name after watching Grease so many times. In the final scene, Sandy and Danny dance on an amusement park ride called the “Shake Shack” while singing “You’re the One That I Want.”

3. BUT IT WAS ALMOST CALLED "CUSTARD'S FIRST STAND."

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Meyer acknowledged in a 2015 interview with the New York Times that the name was “pretty bad.”

4. ITS INSPIRATION IS DISTINCTLY MIDWESTERN.

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The St. Louis-raised Meyer had a fondness for the burger joints and frozen custard stands he grew up with—places like Ted Drewes, Steak 'n Shake, and Fitz’s. So when the time came to develop the concept for Shake Shack, he reached back to the crinkle fries and chocolate malts of his childhood.

5. MEYER DREW UP THE MENU IN LESS THAN 10 MINUTES.

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In an interview with Bon Appetit, Meyer said he wrote down the original Shake Shack menu on a napkin in exactly nine minutes. And it proved to be eerily on-target, outlining many of today’s Shake Shack standards. The current CEO, Randy Garruti, has the menu framed in his office.

6. IT OFFERS CONCRETES CUSTOMIZED BY LOCATION.

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Florida locations feature concretes made with key lime tarts from Palm Beach’s Sugar Monkey bakery, while Philadelphia Shake Shacks offer one made with strawberry puree, lemon ricotta, and crushed up cannoli shells from Termini Brothers bakery. At the company’s Baltimore location, there’s a custard concrete made using blueberry pancake pie from local baker Dangerously Delicious.

7. IT TOOK THE RESTAURANT EIGHT YEARS TO ADD BACON.

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This would seem like a no-brainer, but Shake Shack, which relies on a meticulous culinary development manager named Mark Rosati to approve new additions, isn’t afraid to take its time. Nowadays, you can get a SmokeShack, or add bacon to any burger.

8. IT'S VERY PICKY ABOUT ITS HAMBURGER MEAT.

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It’s a custom blend of brisket, chuck, skirt steak and short rib made for the company by Pat LaFrieda. Only a few executives know the exact recipe. According to LaFrieda, back in its early days Shake Shack sampled 20 different ground beef combinations before selecting the one they currently use.

9. IT SERVES BREAKFAST, BUT ONLY AT FIVE LOCATIONS.

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That would be the two locations inside New York's JFK Airport’s Terminal 4, the Shake Shack inside New York’s Grand Central Terminal, Washington D.C.’s Union Station location, and, most recently, the original Madison Square Park location. The menu is small, but who doesn't want a breakfast sandwich to help them power through that red-eye?

10. IT'S HAD SOME DELICIOUS SOUNDING MENU FLOPS.

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Like the heirloom tomato custard, or the float made with chocolate custard and stout. There was also a jalapeno and cheddar sausage, which was apparently delightful but had the unfortunate side effect of squirting hot cheese in your face. Whatever: worth it.

11. YOU CAN ORDER A PEANUT BUTTER BACON SHACKBURGER.

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The gooey, meaty concoction ran as a menu item for a short time back in 2010. Apparently it flopped, and Meyer has said there’s no chance of bringing it back (“I draw the line at peanut butter,” he told Bon Appetit). But menu hackers have discovered it exists as a secret menu item, and uses peanut butter mix-in for the shakes.

12. IT OFFERS CORN DOGS THREE TIMES A YEAR.

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They’re all-beef Vienna hot dogs dipped in house-made corn batter and served with sweet relish. And they’re only available on Memorial Day, July 4th and Labor Day.

13. A SHRIMP PATTY BURGER SPARKED ITS LONGEST LINE EVER.

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The line at Shake Shack’s original Madison Square Park location is long most days (you can check ahead using the nifty line cam). But on June 10th of 2014, it was egregiously long—so long, in fact, that it wound through the whole park. The reason: A limited-release David Chang “Momofuku Shrimp Stack” burger that married beef and shrimp patties and was topped with Momofuku Hozon sauce. Demand outstripped the supply of 1,000 burgers, and Shake Shack took to Twitter to apologize.

14. IT'S HUGE IN THE MIDDLE EAST.

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Shake Shack has ventured abroad to countries like England, Turkey and Russia. But its most significant international investment has been in the Middle East, with 20 restaurants in states and countries like Kuwait, Lebanon and the United Arab Emirates. Locals love the stuff, apparently, but there have been supply issues.

15. IT HAS A SECRET MENU.

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Menu spotlights include a grilled cheese sandwich made with the restaurant’s famous potato buns, a protein-style burger wrapped in lettuce, and a root beer float.

16. IT HAS ITS OWN RUNNING CLUB.

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Shack Track and Field is a free community fitness club located at Shake Shacks across the country. The running club hosts community runs on the second Tuesday of every month, the restaurant’s website says.