Every day, 68 million people dine under the iconic golden arches of McDonald’s. If you’re one of them, then you’re probably intimately familiar with one of the most famous offerings on the menu after the McRib and the Big Mac: the Filet-O-Fish Sandwich.

This bestselling entree looks simple enough. It features a fried square of wild-caught fish nestled under a slice of American cheese and a smear of tartar sauce. But it has a contentious history. In fact, Ray Kroc, the owner of McDonald’s who was immortalized in the 2016 movie “The Founder,” didn’t want the fishy offering on the menu. It ended up there anyway for one of the oldest reasons in the world: Kroc lost a bet.

The story starts back in 1962 in Cincinnati, Ohio. That year, an enterprising McDonald’s franchisee named Lou Groen had a problem. His customer base was largely Catholic and abstained from eating meat on Fridays (not to mention during Lent!), and they weren’t buying enough burgers to keep his restaurant afloat. According to the Smithsonian Magazine, on Fridays, Groen pulled in just $75 per day. To combat that lack of sales, he masterminded a beef-free option, and the Filet-O Fish Sandwich was born.