Sheldon: I don't understand why you're not enjoying this. Together, in this car, with my enhanced capabilities, we're like 'Knight Rider.'
Leonard: Except, in Knight Rider, the car isn't a yammering sphincter.
Sheldon: You mock the sphincter, but, the sphincter is a class of muscle without which human beings couldn't survive. There are over 50 different sphincters in the human body. How many can you name?
Leonard: I was wrong. This is exactly like Knight Rider

Raj: You slipped and fell into a robot hand?
Wolowitz: Yes.
Raj: Penis first?
Wolowitz: Yes. Now help me!
Leonard: I'd suggest a lubricant, but I have a feeling you fell on some of that as well.

When you say mistake, do you mean a fortunate mistake like the discovery of penicillin?

Don't worry about the moon. We set our laser to "stun."

Sheldon: I should have brought an umbrella.
Leonard: What for? It's not gonna rain.
Sheldon: I know that, but with skin as fair as mine, moon burn is a real possibility.

Sheldon: Are you asking me or telling me?
Leonard: Telling you?

Sheldon: That's not an apology; that's simply an acknowledgment that I was right.
Leonard: Okay, I'm sorry.
Sheldon: There you go.

Leonard: Is that your dad?
Wolowitz: If she grows any more hair on her face, yes.

Leonard: What am I doing in your spam folder?
Sheldon: I put you there after you forwarded me a picture of a cat playing the piano entitled, "this is funny."

Sheldon: Roommates agree that Friday nights shall be reserved for watching Joss Whedon's brilliant new series, "Firefly."
Leonard: Does that really need to be in the agreement?
Sheldon: Might as well settle it now; it's going to be on for years.

Leonard: You still have a cold?
Raj: Maybe, but I don't care. That's the good thing about NyQuil -- it's like, 10% booze. I call it the "nighttime sniffling, sneezing, coughing, so you can talk to girls medicine."

Leonard: Well, it wasn't my fault.
Sheldon: The implication being that you somehow tripped and fell into her lady parts?

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.