The Hamburger Postulate
The Big Bang Theory Season 1 Episode 5 (10/22/07)
While eating at the Cheesecake Factory where Penny works, the guys run into Leslie Winkle, the girl who previously rejected Leonard. Leslie asks Leonard to play in her string quartet as the cellist, but what she really wants is him.
Sheldon later comes home to find a necktie on Leonard's door which Penny explains to him what it means. Penny seems a little disappointed to hear about Leonard's actions but later congratulates him. When Leonard is confused by her words, he decides he's rather pursue a real relationship with Leslie over an imaginary one with Penny.
When he goes to talk to Leslie at the lab, he finds out she only used him for sex. She tells him she's good till New Years. Leonard tells Penny who's very happy to hear the news.