Penny: What about Howard and Raj? How did he become friends with them?
Leonard: I dunno. How do carbon atoms form a benzene ring? Proximity and valence electrons.
Penny: Sure, when you put it like that.

Leonard: Penny, as a scientist, my job is to figure out why things happen. But I don't think I'll ever understand how someone like me could get to be with someone like you. You know maybe I don't need to understand it, I just need to be grateful. I love you, Penny.

Leonard: What would you be if you were attached to another object by an inclined plane, wrapped helically around an axis?
Sheldon: Screwed?
Leonard: There you go.

Leonard: My point is, while you're spending all this time on your own, building computers or practicing your cello, what you're really doing is becoming interesting. When people finally do notice you, they're gonna find someone a lot cooler than they thought. And for those of you that were popular in high school, it's over, sorry. Thank you. Congratulations.

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

Wolowitz: *After he fixed Sheldon's algorithm* Gee, why can't Sheldon get a friend?

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