The famous pianist and composer Ignaz Moscheles sat next to the 15-year-old boy on the piano bench, about to give a piano lesson as a favor to the boy's father. In less than a minute, Moscheles, a sensation on the continent, lionized in England, one of a handful of pianists vying for that ever-shifting "greatest" title, knew that he was "sitting next to a master, not a pupil." He had encountered prodigies before, but never had he seen anyone like Felix Mendelssohn.