Quotes - season 1
Sheldon: [about donating sperm] What if she winds up with a toddler who doesn't know if he should use an integral or a differential to solve the area under a curve?
Leonard: I'm sure she'll still love him.
Sheldon: I wouldn't.
Leonard: You convinced me. Maybe tonight we should sneak in and shampoo her carpet.
Sheldon: You don't think that crosses the line?
Leonard: Yes... For God's sake, Sheldon, do I have to... hold up a sarcasm sign every time I open my mouth?
Sheldon: You have a sarcasm sign?
Leonard: No! I don't have a sarcasm sign.
Leonard: What did Penny mean, "you'd make a cute couple"?
Sheldon: Well, I assume she meant that the two of you together would constitute a couple that others might consider cute. An alternate, though somewhat less likely interpretation is that you could manufacture one. As in, "Oh, look, Leonard and Leslie made Mr. and Mrs. Goldfarb! Aren't they adorable?"
Leonard: If Penny didn't know that Leslie had already turned me down, then that would unambiguously mean that she, Penny, thought I should ask her, Leslie, out, indicating that she, Penny, had no interest in me asking her, Penny, out; but because she did know that I had asked Leslie out, and that she, Leslie, had turned me down, then she, Penny, could be offering me consolation - "That's too bad, you would have made a cute couple..." - while thinking, "good, Leonard remains available."
Sheldon: You're a lucky man, Leonard.
Leonard: How so?
Sheldon: You're talking to one of the three men in the Western Hemisphere capable of following that train of thought.
Leonard: Well, what do you think?
Sheldon: I said I could follow it, I didn't say I cared.
Leonard: What are you doing?
Sheldon: Every Saturday since we've lived in this apartment, I have awakened at 6:15, poured myself a bowl of cereal, added a quarter-cup of 2% milk, sat on this end of this couch, turned on BBC America, and watched Doctor Who.
Leonard: Penny's still sleeping.
Sheldon: Every Saturday since we've lived in this apartment, I have awakened at 6:15, poured myself a bowl of cereal...
Leonard: You have a TV in your room, why don't you just have breakfast in bed?
Sheldon: Because I am neither an invalid nor a woman celebrating Mother's Day.
Leonard: Sheldon, think this through. You're going to ask Howard to choose between sex and Halo.
Sheldon: No, I'm going to ask him to choose between sex and HALO 3. As far as I know sex has not been upgraded to include high-def graphics and enhanced weapons systems.
Leonard: You're right, all sex has is nudity, orgasms and human contact.
Sheldon: My point.
Sheldon: I was analyzing our lie, and I believe we're in danger of Penny seeing through the ruse.
Sheldon: Simple: If she were to log onto www dot socalphysicsgroup dot org forward slash activities forward slash other, click on "Upcoming Events", scroll down to "Seminars", download the PDF schedule, and look for the seminar on molecular positronium, well then, bibbidi-bobbidi-boo, our pants are metaphorically on fire.
Leonard: Argh, again?
Sheldon: Obviously you're not well-suited for three-dimensional chess. Perhaps three-dimensional Candyland would be more your speed.
Quotes - season 2
Sheldon: Leonard, you have to do something about Penny. She's interfering with my sleep, she's interfering with my work, and if I had another significant aspect of my life, I'm sure she'd be interfering with that, too.
Leonard: Why should I do something? You're the one who introduced her to online gaming.
Sheldon: Yes, but you're the one who said hello to her when she moved in. If you'd simply restrained yourself, none of this would be happening.
Leonard: Why don't you just tell her to leave you alone?
Sheldon: I did! I told her, I texted her, I sent out a very emphatic Twitter, I even changed my Facebook status to "Sheldon Cooper wishes Penny would leave him alone"! I don't know what else to do!
Leonard: Well, what am I supposed to to?
Sheldon: I don't know, but if you don't figure something out, I warn you, I shall become very difficult to live with!
Leonard: You mean up until now we've been experiencing the happy, fun-time Sheldon?
Leonard: I'll go talk to her!
Sheldon: Leonard! [knock-knock-knock] Leonard! [knock-knock-knock] Leonard!
Leonard: What Sheldon? What Sheldon? What Sheldon?
Sheldon: Tell me what you see here.
Leonard: The blunt instrument that will be the focus of my murder trial?
Sheldon: This is Stephanie's Facebook page. Now where it should say "in a relationship", what does it say?
Leonard: Stephanie Barnett is single.
Sheldon: Furthermore, earlier this evening, she threw a digital sheep at some guy named Mike. Who's Mike? Why does he get a digital sheep and not you?
Leonard: It's just her Facebook page! And we've only been seeing each other a few weeks.
Sheldon: You don't see it, do you? We're losing her.
Leonard: OK, I'm going to make this very simple. You... are not in this relationship. I... am! Ergo, you have noooo say in anything that happens between me and Stephanie.
Sheldon: I'm afraid I can't allow that. Pursuant to Starfleet General Order 104, Section A, you are deemed unfit, and I hereby relieve you of your command.
Leonard: General Order 104, Section A does not apply in this situation.
Sheldon: Give me one good reason why not.
Leonard: Because this is not Star Trek!
Leonard: Sometimes, I just mosey on down to the third floor and have cereal with Mrs. Vartabedian.
Sheldon: Really? I've never once been invited to have cereal with Mrs. Vartabedian.
Leonard: She doesn't like you.
Sheldon: That is my spot. In an ever-changing world it is a simple point of consistency. If my life were expressed as a function in a four-dimensional Cartesian coordinate system, that spot, at the moment I first sat on it, would be [0,0,0,0].
Leonard: Do you know what baffles me, Sheldon?
Sheldon: Based on your academic record, a number of things, I would imagine.
Sheldon: What exactly does that expression mean, 'friends with benefits?' Does he provide her with health insurance?
Quotes - season 3
Sheldon: Interesting. Sex works better than chocolate to modify behavior. Wonder if anybody else has stumbled on to that.
Sheldon: Alright, Poindexter, sit down, shut up, and listen.
Leonard: I'm sorry?
Sheldon: Oh, that's how my father always began our football conversations, and if you'd like, after the game I'll take you outside and teach you to shoot close enough to a raccoon that it craps itself.
Sheldon: What are you talking about?
Leonard: The cultural paradigm in which people have sex after three dates.
Sheldon: I see, now are we talking about date, the social interaction, or date, the dried fruit?
Leonard: When we watch Frosty the Snowman, he roots for the sun.
Sheldon: Excuse me, but the sun is essential for all life on Earth. Frosty is merely a bit of frozen supernatural ephemera and a stolen hat - a crime, by the way, for which he is never brought to account.
Leonard: What are we supposed to do now?
Sheldon: The only thing we can do. Watch TV on our phones until the criminals return and bludgeon us to death.