Sheldon: (After Leonard whistles) First warning.

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

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

Stephanie: I don't see anything at all, Sheldon.
Sheldon: Well, you're the doctor, but I am constantly hearing this annoying sound.
Leonard: Me too.
Sheldon: Is it a high frequency whistle?
Leonard: No, it's more of a relentless, narcissistic drone.

Howard: Sheldon, don't take this the wrong way, but, you're insane.
Leonard: That may well be, but the fact is it wouldn't kill us to meet some new people.
Sheldon: For the record, it could kill us to meet new people. They could be murderers or the carriers of unusual pathogens. And I'm not insane, my mother had me tested.

Rajesh: Die, Toaster, Die!

Sheldon: *knock knock knock* Leonard? (x3)
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: I often forget other people have limitations. It's so sad.
Howard: He can feel sadness?
Leonard: Not really. It's what you and I would call condescension.

Penny: Now, honey, it's okay. You don't have to get me anything in return.
Sheldon: Of course I do. The essence of the custom is that I now have to go out and purchase for you a gift of commensurate value and representing the same perceived level of friendship as that represented by the gift you've given me. It's no wonder suicide rates skyrocket this time of year.
Penny: Okay, you know what? Forget it. I'm not giving you a present.
Sheldon: No, it's too late. I see it. That elf sticker says to Sheldon. The die has been cast, the moving finger has writ, Hannibal has crossed the Alps.

Sheldon: All I need is a healthy ovum and I can grow my own Leonard Nimoy.
Penny: Okay, all I'm giving you is the napkin, Sheldon.

Wolowitz: He doesn't do anything for me. If I were going to go that way, I'm more of a Zac Efron kind of guy.
Raj: Oh yeah, like you have a shot with Zac Efron.

Sheldon: I would ask you to find some way to suppress your libido.
Penny: I could think about you.
Sheldon: Whatever works.

Sheldon: I believe the appropriate metaphor here involves a river of excrement and a Native American water vessel without any means of propulsion.

Sheldon: Rock - Paper - Scissors - Lizard - Spock

Sheldon: I'm sorry but I'm not going to watch the Clone Wars TV Series until I've seen the Clone Wars movie. I prefer to let George Lucas disappoint me in the order he intended.