Final answer:
The narrator in The Pianist keeps his mind occupied while hiding by listening to the sound of a piano playing songs from the past.
Step-by-step explanation:
In the book The Pianist, the narrator keeps his mind occupied while he is hiding by listening to the sound of a piano playing songs from the past. The narrator finds solace and beauty in the music, which helps to distract him from the difficult situation he is in. This shows how the power of music can provide comfort and escape during challenging times.