One example of Mozart's musical genius ... when he was 14, he visited Rome and heard the Allegri Misereri sung in the Sistine Chapel. He went back to where he was staying, and wrote the whole piece out from memory, having heard it ONCE. 12 minutes long, for 2 choirs, 9 voices in total. Heard it again a few days later, and only had to fix a couple of mistakes. Hard to imagine.