Initially, after feeling that Penny was not for him, Leonard turned his attention to fellow scientist Leslie Winkle. The first time Leonard asks Leslie out, she rejects him, and his further relationships with her proved to be only short-lived casual sexual encounters that only happened to satisfy Leslie's libido. Leslie briefly started dating Leonard in Season 2, which irritates Sheldon. Leslie and Sheldon get into an argument about String and Loop theories, Leonard sides with Sheldon. Leslie breaks up with Leonard by saying him believing in String theory is a "deal breaker".