Sheldon: Leonard, when that woman moved in three years ago I told you not to talk to her, and now look. We're going to be late for the movies.

Sheldon: I present to you the Relationship Agreement. A binding covenant that in its 31 pages enumerates, illuminates and codifies the responsibilities of Sheldon Lee Cooper - hereinafter referred to as the "Boyfriend" - and Amy Farrah Fowler - hereinafter referred to as the "Girlfriend.
Amy: That's so romantic!
Sheldon: Mutual indemnification always is.

Penny: Sheldon, did you have a Christmas tree?
Sheldon: Oh, yes. We had a tree, we had a manger, we had an inflatable Santa Claus with plastic reindeer on the front lawn. And to make things even more jolly, there were so many blinking lights on the house they induced neighborhood-wide seizures.

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

Sheldon: Hello Penny. I realize you are currently in the mercy of your primitive biological urges. But, as you have an entire lifetime of poor decisions ahead of you, may I interrupt this one?

Howard: Don't you think I should answer the engineering questions? I am an engineer.
Sheldon: By that logic I should answer all the anthropology questions because I'm a mammal.

Sheldon: Let's see. What do I know about Amy? She loves medieval literature. Chaucer's her favorite. And her eyes sparkle when she watches old French movies. And I enjoy how harp music causes her fingers to dance as if she's playing along.
Bernadette: Wow, you really do love her.
Sheldon: I do. Now, let's find the kind of gift that makes her feel small and worthless.

Penny: Sheldon, could I ask you a question?
Sheldon: I would prefer that you not, but I wouldn't go so far as to forbid it.

Sheldon: Oh, by the way, I was watching you sleep for a moment, and I noticed that your snoring seems to be worse when you're on your back.
Penny: Leonard doesn't snore.
Sheldon: I wasn't talking to Leonard.
Leonard (to Penny): Told ya.

Sheldon: Amy, when I look in your eyes and you're looking back in mine, everything feels not quite normal. Because I feel stronger and weaker at the same time. I feel excited and, at the same time, terrified. The truth is, I don't know what I feel, except I know what kind of man I want to be.
Amy: Sheldon, that was beautiful.
Sheldon: I should hope so, that's from the first Spider-Man movie.
Amy: I'll take it.

(Sheldon notices Penny taking Leonard to his bedroom)
Sheldon: What's going on?
Penny: Get your noise-cancelling headphones, 'cause it's gonna get loud.
Sheldon: Oh, not this again.

Leonard: Can I have a napkin.
Sheldon: I'm sorry, no!
Leonard: But you have a whole bunch of napkins.
Sheldon: Yes, I've moved to a four napkin system, lap, hands, face, and personal emergency. If you like, starting tomorrow, I'll add a guest napkin but I'm afraid there's nothing I can do for you today.
*Leonard grabs a napkin.*
Sheldon: Good luck, that's the face napkin.

Leonard: Have you considered telling her how you feel?
Sheldon: Leonard, I'm a physicist, not a hippie.

Sheldon: If you don't mind, I'd like to stop listening to you and start talking.

Leonard: You'll never guess what just happened.
Sheldon: You went out into the hallway, stumbled into an inter-dimensional portal which brought you 5,000 years into the future, which you took advantage of the advanced technology to build a time machine, and now you're back to bring us all with you to the year 7010, where we transported to work at the thinkatorium by telepathically controlled dolphins?
Leonard: No.
Sheldon: Aww.
Leonard: Penny kissed me.
Sheldon: Well, who would ever guess that?