For 12 seasons of The Big Bang Theory, the apartment building of Sheldon, Leonard and Penny was one of the most important sets. Outside of their respective apartments, multiple scenes over the years also took place in the hallway, in the building's lobby and even in the downstairs laundry room. But the staircase of the building was also an important location for many scenes, as the trio and their friends Raj, Howard, Bernadette and Amy had to walk up and down three flights of stairs to get to and from the apartments.
These long walks up and down the stairs were the subject of many jokes and important scenes throughout the years, and that was all because the building's elevator was broken. The condemned elevator doors on all four floors of the building became an unquestioned piece of The Big Bang Theory's DNA; however, there's a deeper backstory to the prop.
The origin of the broken elevator was revealed in the Season 3 episode, "The Staircase Implementation." In the 2010 episode, Leonard and Sheldon had yet another fight when the former attempted to adjust the temperature in the apartment, something that went against the famed "roommate agreement." Leonard stormed away and found refuge at Penny's, where he cooled off by telling her how he first moved into the building and became its newest resident.
The episode then flashed back to the year 2003 to show how Leonard first visited the apartment building to pass several tests that would lead to him becoming Sheldon's new roommate. Not only did the episode show how the two best friends first met, but it also revealed how Sheldon adopted his signature spot on his living room couch, and how Howard, Raj, Sheldon and Leonard became a quartet of friends. But perhaps most importantly, the episode revealed how Leonard broke the elevator.
When Leonard first visited the apartment building, the elevator was still working. But briefly after he moved in with Sheldon, he brought one of his experiments home: fuel that he planned on using for a rocket Howard was working on. Unfortunately, Leonard made a mistake when mixing the fuel, not properly adjusting his calculations according to the rocket's dimensions. As a result, the fuel began to smoke inside its casing unit, and a detonation was all but assured. Leonard tried to rush the fuel out of the apartment, but Sheldon saved his life by placing the container inside the open elevator, and closing the doors just in time before the fuel exploded.
As a result, the elevator was broken, and it remained that way until it was finally fixed in the Season 12 finale, "The Stockholm Syndrome." The story of how the elevator was broken revealed one of the show's biggest mysteries, but it also served as a reminder for Sheldon that despite his many, many quirks, he saved Leonard's life, and he didn't rat him out to anyone. And that is a true testament of their friendship.
The Big Bang Theory aired on CBS from 2007-2019 and starred Jim Parsons, Johnny Galecki, Kaley Cuoco, Simon Helberg, Kunal Nayyar, Melissa Rauch and Mayim Bialik. The show is currently streaming on HBO Max.