The Staircase Implementation

Summary
Leonard tells Penny the story of how he first met Sheldon. The meeting involved an elevator. This episode is mostly in flashback. Leonard starts out with explaining how Sheldon's previous roommate ran out and warned him about Sheldon's idiosyncrasies. Leonard runs into their transvestite neighbor and eventually Sheldon. Sheldon quizzes Leonard about science fiction trivia questions and remarks and allows him to enter after he answers correctly. Sheldon gives a series of interview questions to verify whether or not Leonard is a good and worthy roommate to have. After working out seemingly countless pages of the never-before-seen "roommate agreement," Leonard moves in. Sheldon kicks Joyce Kim, Leonard's girlfriend, out of the apartment, which turned out to be a good thing as Kim was a North Korean spy and Leonard had been working on a top-secret rocket fuel for the government. Over time, he replaces Sheldon's lawn chairs with the beloved sofa (prompting Sheldon to find his "spot") and meets Raj and Howard who work at the university. When they discover that Howard has a rocket, Leonard uses his rocket fuel and attempts to launch it. He does not do the math correctly, however, and Leonard attempts to take it outside via the elevator. Sheldon sends it down, and a second after the doors close a loud explosion takes place, explaining why the elevator no longer works. Leonard explains his continued friendship with Sheldon by pointing out that Sheldon saved his life and didn't rat him out to the landlord, the police, or Homeland Security.

Title reference: After a failed experiment renders the elevator useless, the staircase has to be used (and is therefore often featured as a setting for conversations between various characters).

Trivia

 * In this episode, the hitherto unseen characters of Joyce Kim, a North Korean spy who dated Leonard, and the pairs' original neighbour, the "two hundred pound transvestite with a skin condition" are seen.
 * Sheldon makes two blatantly wrong predictions in flashbacks: Firefly would run "for years" and the Apple iPod would dwindle in popularity after the release of the competing Microsoft MP3-Player.
 * In Howard's room of 2003 they showed a Roboraptor and a slimline PS2, neither of which was available then.