Here is a little test for your powers of observation. However it will help (a lot) if you have some musical knowledge.
The young man in the video is playing Beethoven's Für Elise on the piano. I should say right at the outset that there is no video trickery, no post production magic. If you had been standing there in the room at the time this piece of music was being played, you would have seen exactly what you see here on the video. So... what is unusual about what is happening?
The answer is on the next page, but see if you can spot what is happening, before you turn the page.