Let’s raise a birthday toast to Mick Jagger, who turns 75 today — over 50 years after he sang “The lines around my eyes are protected by a copyright law” on Aftermath. After all these years in the public eye, he remains the ultimate rock & roll trickster. He’s the most visible of rock stars, yet also one of the most mysterious and unfathomable — not to mention the funniest. He’s the most elusive of the Rolling Stones — anybody can tell right away what’s cool about Keith or Charlie or Woody, but Mick takes pride in keeping his secrets to himself. That’s a key reason why we obsess over him. “Pleased to meet you! Hope you’ve guessed my name!” he leers in one of the Stones’ most infamous classics, “Sympathy for the Devil,” and he’s never made that guessing game easy. Who could hang a name on him? He’s the “Jigsaw Puzzle” none of us will ever solve. Bless him for that.