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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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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The Big Bang Theory Season 4 Quotes

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

Sheldon: Here's an interesting fact about alcohol ... Man is not the only species that ferments fruit in order to become intoxicated. Can you guess what the other is? Hint: sometimes they pack the alcohol in their trunks.
Penny: Monkeys
Sheldon: When does a monkey have a trunk?
Penny: When a suitcase just won't do
Much later - Sheldon: And the answer was elephant.