Only marginally related, but I always wondered why we tip servers a percentage of the meal cost, instead of a flat range based on the service. Is a server at an inexpensive diner really in a lower-skilled position than a server at a cloth-napkin restaurant (enough to justify a $3 tip vs. a $20 tip)? Or is it based on the expectation that a lower cost restaurant will have a larger number of patrons than a more expensive restaurant?