The Big Bang Theory Episode 3.22 The Staircase Implementation

In this episode we trace the history of Sheldon and Leonard and the guys. Penny asks Leonard why he ever moved in with Sheldon, and through flashbacks we learn the whole genesis of their friendship. It started 7 years ago when Leonard was simply looking for a roommate. Sheldon had advertised at the University, and Leonard went to the apartment to meet him. Upon arriving there, he met the old roommate, who told him to "run for you life!" In the apartment Leonard saw that the old roommate has scrawled "Die Sheldon, Die" on the walls. This is where we learn about the roommate agreement, and how Howard and Raj came into the fold. In the end, we find out that Sheldon actually saved all of their lives, by putting an experiment that Leonard was doing in the elevator, and shutting the doors, thereby making sure that Leonard was not near the explosion.