The concerto is by tradition a showoff piece, and Mozart, who was terrifyingly agile on the keyboard and spent his childhood using it to dazzle kings and princes of the church, wrote himself plenty of vehicles. But by the time he was working on his 24th Concerto, K. 491, he had deeper concerns than mere virtuosity. It's in C minor, the key of turbulence, and in it you can hear the incipient romantic revolution that propelled Beethoven into his own C minor concerto 17 years later, a piece clearly modeled on Mozart's.