Anyone who's been to a high school football game will recognize Blast! immediately. The brass-and-percussion band, the routines with flags and batons, the rendition of "Malaguena"--this is is a halftime show. An evening-length halftime show, without a game to justify it. The concept seems perverse. The halftime show rates right up there with ice ballet as one of the strangest forms devised--a weird little cul-de-sac of idiosyncratic skills and odd aesthetics. Thanks to my son the football player, I've been watching high school halftime shows for two years now, and I still can't figure out why musicians should have to learn how to play while walking backward or what distinguishes a cheerleader from one of those girls with the big semaphore flags. But in a way that's what I love about Blast!: it demystifies the halftime show, not by explaining it, but by drawing us into its absurd heart. Blast! is the Platonic ideal of halftime shows. The apotheosis of halftime shows. The halftime show that inhabits the imagination of every marching-band director and pom-pom squad captain and tuba player in America. And yes, it's kind of amazing. With a cast of 42 astonishingly proficient young musicians and dancers, it's at once bombastic and delicate, hokey and sharp. A prime example: the Blast! version of Aaron Copland's Appalachian Spring includes a play on a military drill in which the traditional wooden rifles have been transmuted--if not transcendentalized--into outsize leaves of grass. There's still plenty that's hard to take, though. The "Simple Gifts" portion of Appalachian Spring is, bizarrely, far too busy; after the first hour the brightness of it all can become tedious; and given the cast's collegiate good looks, a "Marimba Spiritual" evokes nothing so much as spring break. Still, Blast! does supply the elation that's supposed to be the essence of halftime. Ford Center for the Performing Arts, Oriental Theatre, 24 W. Randolph, 312-902-1400. Through December 22: Thursday, 7:30 PM; Friday, 8 PM; Saturday, 2 and 8 PM; Sunday, 2 PM. $25-$70.