You're looking at a photo from "The Cohabitation Formulation." Raj's sister returns during the episode.
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Yes, that's Keith Carradine on The Big Bang Theory. He's played the character of Penny's dad.
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Leonard tries to keep an affair secret from Sheldon on "The Irish Pub Formulation." This is a scene from that episode.
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A flashback to when Leonard first moved in with Sheldon and they signed the roommate agreement.
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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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Sara Gilbert is seen here on The Big Bang Theory. She plays the character of Leslie.
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Penny and Leonard come outside to see Sheldon trapped by a net from the security system they installed.
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TBBT Quotes

Mrs. Cooper: Shelly! I'm so glad you're here!
Sheldon: I saw you having naked sex.

Penny: I give up, he's impossible
Sheldon: I can't be impossible, I exist. I believe what you meant to say is, "I give up, he's improbable."