The Lizard-Spock Expansion - S2-E8
Sheldon: What happened?
Leonard: Howard's at the Mars Rover lab. He says he's in trouble. Defcon 5.
Sheldon: Defcon 5? Well, there's no need to rush.
Leonard: What?
Sheldon: Defcon 5 means no danger. Defcon 1 is a crisis.
Leonard: How can 5 not be worse than 1?
Raj: Yeah, Star Trek V, worse than I.
Sheldon: Okay, first of all, that's a comparison of quality, not intensity. Secondly, Star Trek I is orders-of-magnitude worse than Star Trek V.
Raj: Are you joking? Star Trek V is the standard against which all badness is measured.
Sheldon: No, no, no. Star Trek V has specific failures in writing and direction, while Star Trek I fails across the board, art direction, costuming, music, sound editing.
Leonard: Can we just forget I said Defcon and go?
Raj: Star Trek V!
Sheldon: All right, will you at least stipulate that Star Trek IV: The Voyage Home is in-arguably the best?
Raj: I have three words for you. Wrath of Khan.
The Lizard-Spock Expansion - S2-E8
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.
The White Asparagus Triangulation - S2-E9
[Sheldon points to a computer.]
Sheldon: Tell me what you see here.
Leonard: The blunt instrument that will be the focus of my murder trial?
The Lizard-Spock Expansion - S2-E8
Howard Wolowitz: If it's "creepy" to use the Internet, military satellites, and robot aircraft to find a house full of gorgeous young models so I can drop in on them unexpected, then FINE, I'm "creepy."
The Vegas Renormalization - S2-E21
Sheldon: While my compatriots are in Las Vegas, I will be enjoying a blissful evening in my personal Fortress of Solitude.
Penny: That's Superman's big ice thingy, right?
Sheldon: You know, I'm in such a good mood, I'm actually finding your tenuous grasp of the English language folksy and charming today.
The White Asparagus Triangulation - S2-E9
Sheldon: Hi Stephanie, I'm sorry I'm late but your companion left me the most undecipherable invitation
Leonard: What invitation?
Sheldon: [From note.] "We're going to the movies" What movie? What theater? What time? If you wanted to make it impossible to find you, you couldn't have done a better job.
Leonard: Oh I could have.
The Panty Piñata Polarization - S2-E7
Sheldon Cooper: Woman, you're playing with forces beyond your ken.
Penny: Your Ken can kiss my Barbie.
The Lizard-Spock Expansion - S2-E8
Raj: I'll tell you what, how about we go rock-paper-scissors?
Sheldon: Ooh, I don't think so. No, anecdotal evidence suggests that in the game of rock-paper-scissors, players familiar with each other will tie 75 to 80% of the time due to the limited number of outcomes. I suggest rock-paper-scissors-lizard-Spock.
Raj: What?
Sheldon: It's very simple. Look, scissors cuts paper. Paper covers rock. Rock crushes lizard. Lizard poisons Spock. Spock smashes scissors. Scissors decapitates lizard. Lizard eats paper. Paper disproves Spock. Spock vaporizes rock. And as it always has, rock crushes scissors.
The Vegas Renormalization - S2-E21
Sheldon: That's preposterous. I do not resemble C-3PO. Don't get me wrong, I'm flattered. I just don't see it.
The Hofstadter Isotope - S2-E20
Raj: Come on! You know I can't talk to women unless I'm lit up like the Hindu festival of Diwali.
The Hofstadter Isotope - S2-E20
Leonard: Oh hey, Stuart. This is Penny. She's looking for some comic books.
Stuart: Oh, really? Blink twice if you're here against your will.
The Lizard-Spock Expansion - S2-E8
Wolowitz: I got the Mars rover stuck in a ditch.
Sheldon Cooper: Where?
Wolowitz: On a dusty highway just outside of Bakersfield. Where do you think? On Mars!
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.