Everybody has a date. Even you Mario, going after Princess Peach. What am I doing? I'm just enabling you.

Sheldon: Why should I leave? This is my apartment, too.
Leonard: I know it is. And if science ever discovers a second member of your species, and you two would like some privacy, I'll be more than happy to get out of your way

Leonard: How could you just sit there and let them spy on me?
Sheldon: They were clever, Leonard. They exploited my complete lack of interest in what you were doing

Sheldon: You're asking me to keep a secret?
Penny: Yeah.
Sheldon: Well, I'm sorry, but you would have had to express that desire before revealing the secret, so that I could choose whether or not I wanted to accept the covenant of secret-keeping. You can't impose a secret on an ex post facto basis.
Penny: What?
Sheldon: Secret-keeping is a complicated endeavor. One has to be concerned not only about what one says, but about facial expressions, autonomic reflexes. When I try to deceive, I myself have more nervous tics then a Lyme disease research facility.... It's a joke. It relies on the homonymic relationship between "tick," the bloodsucking arachnid, and "tic," the involuntary muscular contraction. I made it up myself

Sheldon: So, you're saying that friendship contains within it an inherent obligation to maintain confidences?
Penny: Well, yeah.
Sheldon: Interesting. One more question—and perhaps I should have led with this—when did we become friends?

Leonard: What do you mean, you're moving out? Why?
Sheldon: There doesn't have to be a reason.
Leonard: Yeah, there kinda does.
Sheldon: Not necessarily. This is a classic example of Münchhausen's Trilemma. Either the reason is predicated on a series of sub-reasons leading to an infinite regression, or it tracks back to arbitrary axiomatic statements, or it's ultimately circular, i.e. I'm moving out because I'm moving out.
Leonard: I'm still confused.
Sheldon: Leonard, I don't see how I could have made it any simpler

Penny: Has [Leonard] ever been involved with someone who wasn't a brainiac?
Sheldon: Oh, well, a few years ago, he did go out with someone who had a Ph.D. in French Literature.
Penny: How is that not a brainiac?
Sheldon: Well, for one thing, she was French. For another, it was literature

Howard. You do not have a PhD. Your cologne is an assault on the senses, and you're not available for video games during the Jewish high holidays

What part of an inverse tangent approaching an asymptote don't you understand?

A fear of heights is illogical. A fear of falling, on the other hand, is prudent and evolutionary

Sheldon [looking at the women's book in a bookstore]: "Coping with the Death of a Loved One." My condolences
Woman: Thank you
Sheldon: Family or friend?
Woman: Family
Sheldon: Too bad. If it had been a friend, I'm available to fill the void

Wolowitz: You know, I'm really glad you decided to learn Mandarin.
Sheldon: Why?
Wolowitz: Once you're fluent, you'll have a billion more people to annoy instead of me

TBBT Quotes

Sheldon: Why do you have the Chinese character for "soup" tattooed on your right buttock?
Penny: It's not "soup," it's "courage."
Sheldon: No it isn't. But I suppose it does take courage to demonstrate that kind of commitment to soup.
Penny: How'd you see it? You said you wouldn't look.
Sheldon: Sorry. As I told you, the hero always peeks.

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.