Kaley Cuoco doesn't appear in this episode because she broke a leg in a horse-riding accident.
This is the first episode in which Amy and Bernadette both appear.
Leonard: (talking about Howard) You realize he's just rubbing our noses in the fact that he has a girlfriend and we don't?
Rajesh: You mean you don't.
Leonard: You have a girlfriend? How could you a girlfriend, you can't even speak to women?
Rajesh: Two words... deaf chick! It doesn't matter if I can't talk because she can't hear me.
Rajesh: That's what she said.
Leonard: All right, well let me see if I can explain your situation using physics. What would you be if you were attached to another object by an inclined plane wrapped helically around an axis?
Leonard: There you go!
Leonard: You changed the address on the building, what about mail?
Sheldon: No worries, I explained my predicament to our letter carrier. He was sympathetic, his exact words were, "Got your back, Jack, bitches be crazy"