It was a full-blown fiesta, complete with a mariachi band, platters of food and boisterous children. No westerner who takes clean, drinkable tap water for granted could possibly have guessed the reason for the Jan. 13 party - the "inauguration" of a water filter designed by Cornell engineering students. The stacked rapid sand filter, developed by members of Cornell's AguaClara research team, could well be the reason that Tamara, a working-class town of about 3,500, now has some of the best water in all of Honduras.