Penny: Can I tell you a secret?
Raj Koothrappali: Yeah.
Penny: I screwed up. Leonard's a great guy. Never should have broken up with him.
Raj Koothrappali: Well er, to paraphrase Shakespeare - It's better to have loved and lost than to stay home every night and download increasingly shameful pornography.
Penny: What happened to Leonard?
Sheldon Cooper: Same thing that happened to Homo erectus. He was replaced by a superior species.
Raj Koothrappali: I'm the new Homo in town!
Raj Koothrappali: That came out wrong.
Sheldon Cooper: What are you doing here?
Leonard Hofstadter: What?
Sheldon Cooper: I said, what are you doing here?
Leonard Hofstadter: I live here.
Sheldon Cooper: I have paperwork that says differently.
Sheldon Cooper: The apartment has three emergency exits located here, here, and here. In the event of a power outage, luminescent paint will guide you to the nearest exit.
Raj Koothrappali: You're kidding.
[Sheldon turns off the lights, revealing glowing arrows on the floor]
Sheldon Cooper: I never kid about safety.
Penny: [to Raj after Leonard leaves] Explain something to me. You watched Leonard put up with that guy for years and years. What has to break inside your brain for you to think: "Oh, Krishna, I've got to get me some of that?".
Sheldon Cooper: Man is not the only species that ferments fruit in order to become intoxicated. Can you guess what the other is? A hint: sometimes they pack the alcohol in their trunks.
Sheldon Cooper: When does a monkey have a trunk?
Penny: When a suitcase just won't do.
[Penny and Raj laugh]
Sheldon Cooper: All right. It would appear as if alcohol is playing keep-away with your intelligence.
Penny: Hi, you guys ready to order?
Sheldon Cooper: [the gang mutters consent] Sure.
Penny: 'Kay. Priya?
Priya Koothrappali: I'll have the Shepherd's pie.
Priya Koothrappali: You want to split that with me?
Penny: Oh, no, no, no, he doesn't.
Priya Koothrappali: Why not?
Penny: Well, you know, milk in the taters, milk in the gravy, parmesan crust... your lactose-intolerant boyfriend will turn into a gas-filled Macy's day balloon.
Howard Wolowitz: You gotta like this. The girlfriend, the ex-girlfriend bonding over your rooty-tooty stinky booty.
Leonard Hofstadter: Kill me.
Sheldon Cooper: It wouldn't help. The human body is capable of being flatulent for hours after death.
[Bernadette is getting her Ph.D]
Penny: Wow! So that means you're a doctor, you're a doctor, you're a doctor, you're a doctor... and Howard, you know a lot of doctors!
Sheldon Cooper: I must say, Amy, I was very impressed to see that Bernadette got her Ph.D.
Amy Farrah Fowler: It's indeed admirable. Although it is microbiology.
Sheldon Cooper: Your doctorate is in neurobiology. I fail to see the distinction.
Amy Farrah Fowler: I'll make it simple for you. I study the brain, the organ responsible for Beethoven's "Fifth Symphony." Bernadette studies yeast, the organism responsible for Michelob Light.
Leonard Hofstadter: Does Sheldon know you're sleeping in here?
Raj Koothrappali: Are you kidding? He made me sign a waiver, participate in an emergency fire drill, and take a refresher course in CPR. Thank God he had a dummy.
Leonard Hofstadter: Oh yeah, Mouth-to-Mouth Mona. You know she used to date Howard?
Penny: [after Penny drunkenly sleeps with Raj, she tries to sneak out, but is finds Sheldon, Leonard, and Howard in the living room] Damn.
Leonard Hofstadter: What is going on?
Penny: [laughing nervously] Oh, it's, it's not what it looks like.
[embarrassed, Penny quickly leaves]
Sheldon Cooper: What does it look like?
Raj Koothrappali: What's wrong with me, Penny?
Penny: Nothing, nothing. You know, if we weren't friends - and you hadn't brought up that creepy pornography story - I'd be on you like the speed of light squared on matter to make energy.
Raj Koothrappali: Hey, you totally got that right! E equals M C squared.
Penny: I listen. I have no idea what it means, but I listen.
Leonard Hofstadter: Do you really think you should be eating that cake?
Howard Wolowitz: Why?
Leonard Hofstadter: If you're gonna be a trophy husband for a rich wife, you might want to watch your waistline.
Raj Koothrappali: He's right. A minute on the lips, a lifetime on the hips.