Leonard tries to keep an affair secret from Sheldon on "The Irish Pub Formulation." This is a scene from that episode.
Added:
Amy has a request of Sheldon on the episode "The Desperation Emanation." She wants to introduce him to her mother.
Added:
That's Katee Sackhoff and that's George Takei. Both guest star as themselves on the episode "The Hot Troll Deviation."
Added:
Who needs human companionship?!? Not Sheldon, this character hopes, on the episode titled "The Zazzy Substitution."
Added:
How can Sheldon live longer? That's what he tries to figure out on the episode "The Cruciferous Vegetable Amplification."
Added:
Sheldon requires a chaperon on his first date when season four of The Big Bang Theory kicks off. Penny is chosen for the job.
Added:
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?
Added:
Science pays off for Wolowitz on the season four premiere. He gets some solid use out of his robotic arm.
Added:

The Big Bang Theory Season 4 Quotes

Leonard: Hi. I'm Leonard. You are beautiful. You pop, sparkle and buzz electric. I'm going to pick you up at eight, show you a night you will never forget.
Raj: Where are we going?

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