“He owed it all to a stolen bicycle,” Barretta writes in this playful, dynamic look at the champion’s quest for greatness. We see how one childhood incident – the young Cassius Clay reported the theft to a police officer, who invited him to learn to box – set the stage for a long career. There are highlights of his boxing fame, later years and racial-justice and humanitarian work. Morrison (“The Quickest Kid in Clarksville”) gives the art a joyful zing and a serious yet eminently kid-friendly vibe.