Marilyn Megenity's eclectic and artful restaurant and nightclub is an unofficial home to many of the city's social and professional dancers. Nearly every night of the week, you can catch a class and/or performance from someone who's more than willing to swivel her belly, hop the lindy or swing all over the Merc's wooden floor. But more than any of the club's other weekly offerings, the Friday-night Tango events have begun to make the café's mercury rise. The sultry Argentinian dance is gaining in popularity what it may lack in accessibility: Dancers say it takes years of careful practice to perfect even the most basic tricks of the tango trade. Which means the Mercury staff and curious observers can count on seeing the same saucy customers sauntering around the floor in their high heels week after week.