Well, it's quite a thread--tipping--on the mileage forum, at that! The only thing I can suggest to add is to rent Reservoir Dogs and take in the "tipping scene", which occurs early in the movie. For me, it was one of the more memorable treatises on tipping I have come across.

Collateral dialogue (language) during the scene (as well as in most of the rest of the movie) may be too intense for pre-teenagers.