The "Moonlight Sonata," by Beethoven, is one of the most beloved piano compositions of all time. There is an interesting story about its creation which is explained by the Beethoven-Haus (Beethoven House) in Bonn, Germany:
In the middle of the 19th century there arose a striking fable surrounding the composition of Beethoven’s "Moonlight Sonata." Beethoven meets a blind girl sitting at a piano and, stricken by her fate, he sits down himself at the piano and can suddenly feel the rays of moonlight coming in through the window and weaving themselves into the notes he is playing. He rushes home and writes down the "Moonlight Sonata."
In another version he watches how the moonlight is reflected in the features of a blind girl while he is playing for her and her brother. The tale and its symbolism were of particular interest to visual artists.
It is, alas, just a fable according to Beethoven scholars.
In this video clip, Daniel Barenboim plays one of Beethoven's most-famous works for piano.
Embedded video online via YouTube.