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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TBBT Quotes

You can't ruin a friendship with sex that's like trying to ruin ice cream with chocolate sprinkles.

Raj

Sheldon: Even the dung beetle chooses to plot its course by using the Milky Way.
Raj: Is that true?
Sheldon: Everything I say is true. Now, of course, the dung beetle also enjoys eating fece, living in feces and making little balls out of feces. So, pick and choose which aspects of its lifestyle you want to embrace.