Here are the plans for the original Shake Shack scribbled on a napkin

Shake Shack started out as a hot dog cart in New York City's Madison Square Park.

Three years into serving hot dogs, restaurateur and Shake Shack founder Danny Meyer came up with an idea to turn the cart into a permanent burger stand, and he sketched the plan onto the back of a napkin.

That sketch laid the blueprint for Shake Shack's original location in Madison Square Park. Ten years later, the burger chain is now a billion-dollar business with more than 60 locations across the US.

Here's the napkin that started it all, courtesy of Shake Shack:

Shake Shack CEO Randy Garutti explained the story behind the napkin sketch to Forbes earlier this year. Here's what he said:

"For three years, we ran the hot dog cart, never thinking beyond that. Then, the city was fielding proposals to fill a 400 square foot kiosk in the Park. On that day, Danny sat down and scribbled on the back of a napkin his vision of a modern version of a roadside burger stand. Not retro, not '50s music playing, but today's version of what made that place great. All we ever wanted to do at Shake Shack was to have good people serving fresh food, and to sell a few hot dogs. We put the burger on the menu and the rest is history."