|From an episode of The Big Bang Theory:
Sheldon: Wait! You bought me a present?Heh. I identify, but thankfully don't feel entirely obligated to reciprocate. #psychology
Sheldon: Why would you do such a thing?
Penny: I don't know. 'Cause its Christmas?
Sheldon: Oh, Penny. I know you think you are being generous, but the foundation of gift giving is reciprocity. You haven't given me a gift. You've given me an obligation.