Avengers: Endgame is a cultural phenomenon. The movie, which is now the second highest grossing movie of all time, has been captivating audiences for a solid week now as hardcore Marvel Cinematic Universe fans go in for a second viewing, trying to see if they can spot anything they missed the first time around. With that being said, there are still a few things, like specific pieces of dialogue origins, that fans are trying to figure out. Thankfully, the Russo Brothers and Christopher Markus and Stephen McFeely have been shedding some light on the secrets. There are MAJOR SPOILERS for Avengers: Endgame below.

In Avengers: Endgame, Tony Stark and Pepper Potts have a five-year old daughter named Morgan. She is clearly the light of Stark's life after everything that went on with the Decimation and Thanos' destruction of the Infinity Stones. During a touching scene where Stark is putting Morgan to bed, he says, "I love you tons," and Morgan replies with, "I love you 3,000." The line has quickly become an MCU fan-favorite, but it wasn't written by Christopher Markus and Stephen McFeely. Markus explains.

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"Well, as much as we'd like to take credit for what is inevitably going to be one of the most memorable lines in MCU history, that is something that Robert and his children actually say to each other. He brought it from real life onto the set."

"I love you 3,000" is something that Robert Downey Jr. and his children say to each other and now it's a part of MCU history. The touching line is brought up again at the end of Avengers: Endgame, solidifying its place in the history books. Bootleg Marvel Studios merchandise has already popped up online and one can easily imagine the studio will have its own take on the dialogue in the near future.

Robert Downey Jr.'s other famous line in Avengers: Endgame was surprisingly not brought to the table by himself, the Russo Brothers, or Christopher Markus and Stephen McFeely. Instead, it was a last minute addition by editor Jeff Ford, who suggested Tony Stark recite his iconic line from the end of 2008's Iron Man, "I am Iron Man," while taking down Thanos. The Russos loved the idea so much that they got ahold of Downey Jr. and shot it the next day to secure its place in the final cut of the movie.

Avengers: Endgame is breaking box office records left and right as of this writing. James Cameron's Avatar is the highest grossing movie of all time, but it's looking like Endgame may end up taking that achievement away in the near future, which is pretty remarkable. Not even Marvel Studios predicted the movie would resonate so much with the world and MCU fans. Regardless, the movie is continuing to sell out screenings and bring in new fans. The interview with Christopher Markus and Stephen McFeely was originally conducted by Business Insider.