(Penny is sitting on the couch, in Sheldon's spot)
Sheldon: You're in my spot.
Penny: Are you planning on sitting here?
Sheldon: No, I'm going to the comic book store.
Penny: Then what difference does it make?
Sheldon: (incredulous) "What difference does it make?"
Leonard: Here we go.
Sheldon: That is my spot. In an ever-changing world, it is a single point of consistency. If my life were expressed as a function on a four-dimensional Cartesian coordinate system, that spot, at the moment I first sat on it, would be (0,0,0,0).
Leonard: Don't sit in his spot.