If you don't have a ready answer to that question, it's not surprising. Getting a glassful of reliably clean, drinkable water every time we turn the tap is something most Americans can happily take for granted. And the costs are buried inextricably in the municipal financial melange: tax revenue, state and federal monies, special appropriations, and ongoing programs begun by past politicians long out of office, during fiscal years long forgotten.

But like near everything else that we get courtesy of planet Earth (which is to say, everything) water really does have a cost -- we're just used to not accounting for it when we're making other economic plans. So this full-page ad in yesterday's edition of The New York Times (yes, I read the paper paper; I'm a sentimentalist) caught my attention: in recognition of World Water Day, dozens of New York restaurants are encouraging today's patrons to pay a dollar for N.Y. tap water. The proceeds will go UNICEF to help provide clean drinking water to millions of children worldwide.

Called Tap Project, it's a canny effort on UNICEF's part: a dollar for a glass of water is a painless act of philanthropy for a diner at Aquamarine, Craft, or Wallse. And I'd like to think it'll also bring on a bit of reflection on the cost -- and the worth -- of our high-quality New York State fresh, reliably drinkable water, as well as how we ought to factor both into planning for a long, sustainable future for our city.

Happy World Water Day!