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, 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: Can't we just go to Big Boy? They only have one burger: the Big Boy. Penny: The Barbecue Burger is like the Big Boy. Sheldon: Excuse me, in a world that already includes a Big Boy, why would I settle for something that's like a Big Boy?
Sheldon: I need your help in a matter of semiotics. Penny: What? Sheldon: Semiotics, the study of signs and symbols as a branch of the philosophy related to linguistics. Penny: Okay, honey, I know you think you are explaining yourself, but you're really not