What is it with musical theater and its ability to make difficult subject matter palatable to the unwashed masses? Think about some of the great hits of the past decade or two: Rent (AIDS), Book of Mormon (a send-up of latter-day saints), Spring Awakening (suicide and abortion in 19th-century Germany), Next to Normal (mental illness). Opening tonight in Raleigh is Urinetown, which in a little more than a decade has traveled from the New York fringe to international infamy and regional war horse status. The premise is something out of Aristophanes: a community living in a historic 20-year drought finds that they're not allowed to pee for free anymore. Songs galore! Be sure to visit the toilet before each act. For some reason, this potty-based show is deemed inappropriate for those under 13.