Penny: *using Sheldon's knock* Sheldon? Sheldon? Sheldon? *Sheldon answers with a suspicious and smiling look on his face*
Sheldon: I bet that started off as a joke, but by the third one, you realized there was something strangely enjoyable about it.
Penny: Yeah, I kind of wanna do it again.
Sheldon: I don't recommend it. You'll be doing it the rest of your life.