Penny: So, what's new in the world of physics?
Leonard: Nothing.
Penny: Really, nothing?
Leonard: Well, with the exception of string theory, not much has happened since the 1930s. And you can't prove string theory, at best you can say “Hey, look, my idea has an internal logical consistency.”
Penny: Ah. Well I'm sure things will pick up.

Leonard: If I take it off, Sheldon wins.
Penny: Sweetie, every night you don't kill him in his sleep, he wins.

Sheldon: Woman, you are playing with forces beyond your ken.
Penny: Yeah, well your Ken can kiss my Barbie.

Leonard: What am I supposed to do?
Penny: Err, keep your mouth off other women.

Howard: Sheldon, if you were a robot and I knew and you didn't, would you want me to tell you?
Sheldon: That depends. When I learn that I'm a robot, will I be able to handle it?
Howard: Maybe, although the history of science fiction is not on your side.

Sheldon: I think that you have as much of a chance of having a sexual relationship with Penny as the Hubble telescope does of discovering at the center of every black hole is a little man with a flashlight searching for a circuit breaker.

Wolowitz: Love is not a sprint, it's a marathon, a relentless pursuit that only ends when she falls into your arms - or hits you with the pepper spray.