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It’s a big week for Big Mac lovers.

To celebrate the release of two new versions of its iconic burger, McDonald’s is giving away 10,000 bottles of its signature Big Mac Special Sauce this Thursday.

Though many have claimed the creamy condiment is nothing more than Thousand Island dressing, McDonald’s insists its Big Mac sauce is truly something special.

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"The Big Mac Sauce is unique to McDonald’s and includes a classic combination of ingredients. The Bic Mac Sauce remains timeless and customers around the world continue to enjoy its signature taste,” McDonald's spokeswoman Becca Hary said in an emailed statement.

McDonald’s has never sold the sauce in the U.S. and the giveaway will be on a first-come-first serve basis at participating locations-- which will be announced Wednesday.

To snag a free bottle, customers must say the code phrase “There’s a Big Mac for that.” The chain also says it will be offering a special bottle number 10,001, to be auctioned off on Friday to benefit the Ronald McDonald House Charities.

McDonald’s introduced the Big Mac in 1968 but it wasn’t until 1975 that the unique condiment got its own callout in the signature Big Mac commercial touting the burger’s components—“Two all-beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.”

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The double-decker burger hasn’t changed much in nearly 50 years but starting this week, the chain began offering two different sized variantions of the classic sandwich nationwide.

A larger version of the classic burger called the “Grand Mac” features two larger beef patties—totaling a third of a pound of meat—plus more cheese and more sauce all piled onto a sizeabley larger bun. The smaller version, "Mac Jr.," is a single patty with the usual Big Mac toppings.