The Big Bang Theory Season 1 - Episode 5

"The Hamburger Postulate"



While eating at The Cheesecake Factory where Penny works, the guys run into Leslie Winkle (Sara Gilbert), a fellow scientist who works with Leonard (and previously turned him down for a date saying she had no attraction to him). She suggests that Leonard play in her string quartet as they are in need of a new cellist. However what she really wants is to get personal with Leonard.

When Sheldon comes home and finds a necktie on Leonard's door, he runs to Penny’s apartment to ask her what the necktie means. Penny tells him and seems slightly disappointed, though she does congratulate Leonard later on. Leonard tries to figure out what her words meant but eventually gives up, deciding he should choose a real relationship (Leslie) over an imaginary one (Penny). However when he arrives in the lab Leslie makes it clear she used him just to satisfy her sex drive, saying she's 'Good till New Year's'. Leonard later tells Penny, who walks away with a smile on her face; she seems to be quite happy to hear this news.
The title refers to Sheldon's unwillingness to try one of the Cheesecake Factory hamburgers due to the fact that he only liked the hamburgers from another restaurant.