Leonard: Can I go back and prevent you from explaining that to me?
Sheldon: Same paradox. If you were to travel back in time and, say, knock me unconscious, you would not then have the conversation that irritated you, motivating you to go back and knock me unconscious.
Leonard: What if I knock you unconscious now?
Sheldon: It won't change the past.
Leonard: But it'd make the present so much nicer.

Leonard: Would someone please turn off the Sheldon commentary track?

Leonard: Sheldon, you do this all the time! You fixate on some crazy idea then blow it way out of proportion.
Sheldon: Name one time I've ever done that.
Leonard: How about the time you put GPS trackers in your garbage because you thought North Korean spies were stealing your doodles? The chicken nuggets you thought were human nuggets. The mysterious cloud that was following you around town. Or the time you put my shirt on by accident and thought you were growing again.
Sheldon: I said name one. You need to work on your listening skills.

Penny: What are we gonna do?
Leonard: We? No, no, no. You had your chance to be "we" for like a year and a half now. Right now, you are you and you are screwed!

Leonard: [to Howard and Raj] Sheldon is at the Cheesecake Factory. [On the phone with Penny] Just tell him to go home.
Penny: Well, he won't leave. He says he's afraid he'll pass out on the bus and someone will harvest his organs.
Leonard: [to Howard & Raj] He is paranoid and he has established a nest.

Stephanie: So, how was your day?
Leonard: You know, I'm a physicist, I thought about stuff.
Stephanie: That's it?
Leonard: I wrote some of it down.

Sheldon: Hard as this may be to believe, it's possible that I'm not boyfriend material.
Leonard: Glad I was sitting down for that.

Leonard: I call Frodo!
Sheldon, Raj, and Howard: Damn!

Sheldon: Everyone at the university knows I eat breakfast at 8:00 and move my bowels at 8:20.
Leonard: Yes, how did we live before Twitter?

Leonard: If Penny didn't know that Leslie had already turned me down then that would unambiguously mean that she, Penny, thought I should ask her, Leslie, out, indicating that she, Penny, had no interest in me asking her, Penny, out but because she did know that I had asked Leslie out and that she, Leslie, had turned me down then she, Penny, could be offering me consolation. "That's too bad, you would have made a cute couple" but while thinking "good, Leonard remains available."
Sheldon: You're a lucky man Leonard.
Leonard: How so?
Sheldon: You're talking to one of three men in the western hemisphere capable of following that train of thought.
Leonard: Well, what do you think?
Sheldon: I said I could follow it, I didn't say I cared.

Raj: Guys, I just did a quick calculation. Judging by the size of the theater and this line we may not get in.
(Sheldon wakes up and jumps out of his chair.)
Sheldon: What did he say?
Leonard: Nice going, Raj. I just got him down for his nap.

Sheldon: Leonard, have you ever wondered why my little toes and lateral incisors are significantly smaller than the average for someone of my size?
Leonard: I wonder a lot of things about you, Sheldon, but not that.

Sheldon: I believe I may have cholera.
Leonard: There's no cholera in Pasadena. Just like last summer when there was no malaria in Pasadena.

Leonard: I'd make fun of Sheldon for having girl problems, if I wasn't shocked that Sheldon has girl problems.

Leonard: Sheldon's nervous about me leaving. Just keep an eye on him while I'm gone.
Penny: Oh, I don't know. Remember what happened when I took care of your goldfish?
Leonard: Well, flush Sheldon down the toilet and get me a new one.