Sheldon: I wouldn't tell you the secret. Sssh!
Leonard: What secret? Tell me the secret.
Sheldon: Mom smokes in the car. Jesus is okay with it, but we can't tell dad.
Leonard: Not that secret, the other secret.
Sheldon: I'm Batman! Ssssh!

Penny: Sheldon, could I ask you a question?
Sheldon: I would prefer that you not, but I wouldn't go so far as to forbid it.

Leonard: How could you just sit there and let them spy on me?
Sheldon: They were very smart. They used my complete lack of interest in what you are doing.

Mrs. Wolowitz What's going on? Are you boys roughhousing?
Howard: We're just talking, ma.
Mrs. Wolowitz If you don't settle down right now, I'm not going to let you have any more sleepovers.

Sheldon: I'm baaaaaaaaaaaaaaaack.

Penny: So, do you think that if Leonard and I keep dating, Leonard would get bored with me?
Sheldon: That depends.
Penny: On what?
Sheldon: Do you have a working knowledge on Quantum Physics?
Penny: No.
Sheldon: Do you speak Klingon?
Penny: No.
Sheldon: Do you know any card tricks?

Sheldon: And you thought the opposite of stupid loser was a Community College Graduate?

Sheldon: When I try to deceive I have more nervous ticks than a lime disease research facility. It's a joke. It relies on a homonymic relationship between the tick the blood sucking arachnid, and tick the involuntary muscular contraction. I made it up myself.

Raj: Tag, you're it!
Howard: Shouldn't you put him in a brown paper bag and set him on fire?

Mrs. Wolowitz:What's going on? Are you boys rough housing?
Howard: We're just talking, Ma!
Mrs. Wolowitz: If you don't settle down right now, I'm not gonna let you have any more sleepovers.
Howard: For God's Sake, Ma, I'm 27 years old. And it's not even a school night.

Sheldon: I drank milk that tasted funny.

Raj: I hate trains.
Sheldon: Oh, don't be silly. You love trains.

Wolowitz: Leonard, how was your date?
Leonard: Bite me!

Leonard: Penny thinks I'm too smart for her. That's ridiculous!
Sheldon: I know! Most of your work is extremely derivative. And don't worry, that's not a secret.

Penny: Look, just forget I told you about me not graduating from community college, okay?
Sheldon: Forget? You want me to forget? This mind does not forget. I haven't forgotten a thing since the day my mother stopped breast-feeding me. It was a drizzly Tuesday.