After their return home from their three-month expedition in the North Pole, Sheldon finds out from Howard and Raj that the guys had tampered with his experiment because Sheldon was absolutely unbearable when he wasn't getting the results he wanted (plans to kill him were even brought up and discussed). They tell him about their use of an electric can opener to give Sheldon positive results, thereby causing Sheldon to believe he has made a Nobel Prize-winning discovery. Unfortunately, Sheldon had immediately emailed everyone at Caltech about his discovery before Howard and Raj could tell him what had happened. Sheldon is so disgraced that he resigns and moves back in with his mother (played by Laurie Metcalf), who lives in east Texas. Mrs. Cooper calls Leonard and tells them that they should come talk to Sheldon. Upon the guys' arrival, Sheldon is intent on staying in Texas. However, he shortly has a disagreement with his mother about creationism versus evolution and decides to return home.
Meanwhile, Penny has been harboring feelings for Leonard, which are released almost as soon as he comes home. Leonard's "moments" with her are constantly interrupted, and he "cannot catch a break." They finally sleep together at the end of the episode (Leonard finally "catches his break") and Leonard asks why it's always weird after friends sleep together and why they have to label it as such - "it just is what it is." Penny interrupts him saying "it's weird" and he replies "Totally."
DOWNLOAD THE BIG BANG THEORY S03E01 - THE ELECTRIC CAN OPENER FLUCTUATION