A chance meeting in the hospital hallway:
Mark: Hey, Jeff.
Jeff: Hey, Mark.
Mark: Do you know who's on call this weekend?
Jeff: I guess it's not you?
Mark: No...and I guess that means it's not you?
Jeff: No. What about Bob?
Mark: No...Bob is on vacation this week.
Jeff: And it can't be Rob, because he was on last week.
Mark: So it has to be Dave!