Big thanks to those of you who responded with your answers.
As expected, they were funny, thought provoking, and on-target.
Now here are some I've heard or come up with over the years.
"Tip money is..."
1. Beer money.
2. A window into the soul of the tipper.
3. An economic thermometer.
4. The only reason I keep coming back.
5. How I can tell if they like my singing.
6. Date money.
7. Sometimes the only thing standing between me and bankruptcy.
8. Often spent foolishly.
9. Better than going to a Redi-Teller.
10. How you can tell the generous clients from the tight-wads.
11. Pays for my parking tickets.
12. Kept in a drawer until I really need it.
13. Buying a round of drinks tonight.
14. To a gondolier, what applause is to an actor.