Leonard join Leslie's string quartet at the cellist. However, we all know the real reason Leslie invited Leonard to join.
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The boys go to eat dinner at the restaurant where Penny works as a waitress and she serves the guys.
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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.
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The Big Bang Theory Season 1 Episode 5 Quotes

I saw your pupils dilate when you looked at her, which, unless you're a heroin addict, points to sexual attraction

Leslie [about Leonard's interest in Penny]

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?