In this penultimate episode from the previous season of "The Big Bang Theory": Penny, doing her nails in her apartment, overhears Leonard arguing with Sheldon about the thermostat which ends with Leonard knocking on Penny's door and asking to sleep on the couch. As she asks how he managed to put up with him, we flashback seven years before when Leonard was looking for a place to live. He had longer hair then and the elevator actually worked at the time. Anyway, before going up, Sheldon's former roommate warns him to stay away. We find out why when we see what Sheldon's apartment looks like: mostly spare with only charts and lawn chairs decorated (and what do you know, Sheldon's chair spot is the same as his later couch one). Also, Dr. Cooper has a lot of rules, barriers, and pertinent questions like "Kirk or Picard?" (Leonard answers that he prefers "The Original Series" over "The Next Generation" but Jean-Luc over James T.) Later on, Leonard invites his new university friends Howard and Raj over for a video game challenge with the eventual permanent couch in tow. And it's here we also find out what happened to make that elevator stop working. Something to do with Howard's model rocket and Leonard's fuel...This origin story of how Leonard and Sheldon became roommates was quite funny what with all the details concerning not only what I cited above but also about Leonard's first time taking a girl to his new place, the previous tenant in Penny's, and how Sheldon saved Leonard's life a couple of times. Also, we briefly find out what Penny did seven years previous. So on that note, "The Staircase Implementation" comes highly recommended.