Of all the things one can stick in one's mouth, the barbecued rib is perhaps the most enigmatic. Its meat-yield-to-eating-effort is prohibitive, it's a messy procedure at the best of times, and ribs always make you thirsty; exactly why are ribs so popular? Don't ponder the ineffable, son just stick one of those smoky-sweet babies in your maw, and find enlightenment. The Rib America Festival (www.ribamerica.com) concentrates some of the nation's most dedicated rib-tenders at Soldiers Memorial Plaza (1315 Chestnut Street) from Friday to today (May 26 through 29). Admission is free before 4 p.m., but you'll need plenty of cash to satisfy your pork needs. And don't fill up on bread or sides: That's a rookie mistake.