*Leonard tries to slip a note under Penny's apartment door*
Penny: Hi.
Leonard: Oh.
Penny: What's going on?
Leonard: Um, here's the thing. (Reads from the note.) Penny, just as Oppenheimer came to regret his contributions to the first atomic bomb, so too I regret my participation in what was, at the very least, an error in judgment. The hallmark of the great human experiment is the willingness to recognize one's mistakes. Some mistakes, such as Madame Curie's discovery of Radium turned out to have great scientific potential even though she would later die a slow, painful death from radiation poisoning. Another example, from the field of Ebola research-
Penny: Leonard.
Leonard: Yeah.
Penny: (hugs him) We're okay.

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: Oh gravity, thou art a heartless bitch.

Sheldon: I have a masters and two PhDs. I should not have to do this.

Penny: Yes, I know men can't fly.
Sheldon: No, no let's assume that they can. Lois Lane is falling, accelerating at an initial rate of 32ft per second, per second. Superman swoops down to save her by reaching out two arms of steel. Ms. Lane, who is now traveling at approximately 120 miles per hour, hits them, and is immediately sliced into three equal pieces.
Leonard: Unless Superman matches her speed and decelerates.
Sheldon: In what space, sir, in what space? She's two feet above the ground. Frankly, if he really loved her, he'd let her hit the pavement. It would be a more merciful death.