Sheldon:  Wait! You bought me a present?
Penny:  Uh-huh.
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.