Three strikes and you’re out. It’s a sports metaphor.

Sheldon: Excuse me, Penny, but we’re…
Leonard: No, no, don’t tell her.
Sheldon: …playing Klingon Boggle.
Leonard: Aw!
Howard: What do you mean, aw? Like she didn’t know we were nerds?

Howard: Oh, look! That's the future Mrs. Wolowitz. No, wait! That's the future Mrs. Wolowitz. With her head in the lap of... what a coincidence... is the future Mrs. Wolowitz.
Leonard: Yeah, and they can all move in with you and your mother. The current Mrs. Wolowitz.

Howard: You’re telling me that I’m within driving distance of a house filled with aspiring supermodels?
Penny: Yeah, I guess.
Howard: And they live together and shower together and have naked pillow fights?

Howard: Every week, they kick out a beautiful girl, making her feel unwanted and without self-esteem, a.k.a. the future Mrs. Howard Wolowitz.

Penny: Aw honey, the buses don't go where you live do they?

Leonard: Congratulations. You’re officially one of us.

Leonard: Yeah, yeah, ah, see here's the thing, after you leave, I still have to live with him.

Penny: You didn't know? You have three strikes. Strike 1- You came in, Strike 2- You sat down and Strike 3- I don't like your attitude.

Penny: No Shirt, No Shoes, No Sheldon.

Penny: It is on. I am gonna introduce your friend to a world of hurt.
Leonard: Oh, Penny, you don't want to get into it with Sheldon. The guy is one lab accident away from being a supervillain.

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

Sheldon: When you understand the laws of physics, Penny, anything is possible. And may I add, "Mwah, ha, ha."

Sheldon: Well played.
Penny: Thank you.
Sheldon: Just remember: with great power comes great responsibility.
Penny: Understood.