Leonard join Leslie's string quartet at the cellist. However, we all know the real reason Leslie invited Leonard to join.
The boys go to eat dinner at the restaurant where Penny works as a waitress and she serves the guys.
When Sheldon discovers a necktie on Leonard's door he runs to Penny to find out what it means. Penny explains to the clueless nerd.

The Big Bang Theory Season 1 Episode 5 Quotes

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