As the Rolling Stones jumped into “Jumping Jack Flash” near the end of their Philadelphia show last Tuesday, I flashed to one thought: this is why I’m here. That riff is what hooked me on the Stones. Keith Richards’s 11 note riff made that song, I bought the single, I got the Stones. More than 50 years later, remembering wearing out the record on a mono record player, I basked in that guitar riff as it washed around me in the delirium of stadium rock, part group ecstasy, part mind-altering acoustical slapback from the concrete bowl.