You're looking at a photo from "The Cohabitation Formulation." Raj's sister returns during the episode.
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Koothrappali is living in a fantasy world. It's bad because he's dancing there with his best friend's gal.
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Former basketball player Rick Fox is seen here on The Big Bang Theory. It's a scene from the episode "The Love Car Displacement."
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Leonard gets an idea for a smart phone app on the first Big Bang episode of 2011. This is a shot from the opening scene.
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The group goes all Justice League in this scene/episode of The Big Bang Theory. Seems about right for the show, doesn't it?
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A picture of Sheldon's love interest, Amy Farrah Fowler, on The Big Bang Theory.
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Sorry, it's not the best picture, but here's a photo of Bernadette from The Big Bang Theory.
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Penny, Bernadette and Amy are seen here during a night out. It's a scene from the episode "The Alien Parasite Hypothesis."
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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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Wil Wheaton returns to The Big Bang Theory on the episode "The 21-Second Excitation." He once again causes problems for the fellas.
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Welcome to comedy, Eliza Dushku. The actress appears as a FBI agent on The Big Bang Theory episode "The Apology Insufficiency."
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Eliza Dushku guest stars on the November 4 episode of The Big Bang Theory. She plays a federal agent.
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TBBT Quotes

Penny: Here's a question-- as an alien pretending to be human, are you planning to engage in any post-prom mating rituals with Amy?
Sheldon: There are post-prom mating rituals?
Penny: Not always. Unless your date drives a van with an air mattress, then always.
Sheldon: Well, if it's part of the prom experience, then I'm open to it.
Penny: You're kidding.
Sheldon: I may be an alien, but I have urges.If Amy wants to copulate by firing her eggs into space, well, then, I will happily catch them with the reproductive sac on my upper flermin. I'm not the best at reading facial cues, but I can see that you're a little turned on.

Penny: Hey, Sheldon, did you change your Wi-Fi password again?
Sheldon: Yes, it's "Penny, get your own Wi-Fi." No spaces.