The Rule Of Reciprocation
Interesting concept here: http://www.npr.org/blogs/health/2012/11/26/165570502/give-and-take-how-the-rule-of-reciprocation-binds-us tl’dr: a study shows if a waiter personally gives an extra piece of candy with the check at the end of the meal, they will receive on average 20% more over their regular tips.