Sheldon: My problem is that I don't always know when I've gone too far.
Leonard: Well, uh, if you like, I could try to help you out. You know, and maybe let you know if you're crossing a line.
Sheldon: Oh, you mean, like, with a code word?
Leonard: Sure. How's "shut up"?
Sheldon: That's perfect. People say it to me all the time, no one will suspect.