A flashback to when Leonard first moved in with Sheldon and they signed the roommate agreement.
Added:
Before there was Leonard, there was an even angrier roommate. Angry enough to write "Die Sheldon Die."
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A flashback to the nerds when they first met playing Gamecube together.
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The Big Bang Theory Season 3 Episode 22 Quotes

Leonard: What am I doing in your spam folder?
Sheldon: I put you there after you forwarded me a picture of a cat playing the piano entitled, "this is funny."

Sheldon: Roommates agree that Friday nights shall be reserved for watching Joss Whedon's brilliant new series, "Firefly."
Leonard: Does that really need to be in the agreement?
Sheldon: Might as well settle it now; it's going to be on for years.