Because he has lived with Sheldon for several years, Leonard knows him better than his other colleagues: this has led Sheldon to believe that Leonard is his best friend. Nevertheless, Leonard often finds himself explaining Sheldon's behavior or serving as a mediator between him and other people in many situations. For instance, when Penny and Sheldon engaged in a fierce dispute, Leonard gave her Sheldon's mother's phone number so she would call her son and scold him for his behavior, thus ending the fight. "The Staircase Implementation" features a flashback to when Leonard first moved in with Sheldon. Leonard explains to Penny that he moved in despite the difficulty of living with Sheldon because the rent was so reasonable. Shortly after moving in, during a rocketry experiment that Leonard, Raj and Howard conducted, Sheldon observed miscalculations Leonard made in mixing the rocket fuel and subsequently saved Leonard's life by pulling him out of the building's elevator just before the fuel capsule exploded. This explains why the elevator is out of order for the duration of the series. Leonard says that Sheldon, despite his tendency to alienate others, kept quiet about Leonard's destruction of the elevator – "He didn't rat me out to the landlord, or the police.... or Homeland Security."