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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Amy has a request of Sheldon on the episode "The Desperation Emanation." She wants to introduce him to her mother.
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That's Katee Sackhoff and that's George Takei. Both guest star as themselves on the episode "The Hot Troll Deviation."
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Who needs human companionship?!? Not Sheldon, this character hopes, on the episode titled "The Zazzy Substitution."
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How can Sheldon live longer? That's what he tries to figure out on the episode "The Cruciferous Vegetable Amplification."
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Say hello to Sheldon's nemesis on The Big Bang Theory: oh, yes, it's Evil Wil Wheaton!
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The Big Bang Theory airs on a new night during its fourth season. But this poster promises the same old hilarity.
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Sheldon requires a chaperon on his first date when season four of The Big Bang Theory kicks off. Penny is chosen for the job.
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Sheldon has a request for Penny on the fourth season premiere of The Big Bang Theory. Will she tag along on a date with him?
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Science pays off for Wolowitz on the season four premiere. He gets some solid use out of his robotic arm.
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Jim Parsons is seen here at The Big Bang Theory Comic-Con panel in 2010. He and his castmates dished on the new season at the event.
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A flashback to when Leonard first moved in with Sheldon and they signed the roommate agreement.
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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.