Have you ever played Guitar Hero? Then you probably also once held a guitar and had no way how to actually use it. Playing the notes as they appear on the screen requires proper hand-eye coordination, but is quite far from learning how to play the instrument the way you're "supposed to". Nevertheless, you do learn how to play to some extent.
Synthesia is software designed to let you learn how to play the piano, without having to go through half a millennium of music theory. It works in the same way as guitar hero does: it displays a keyboard and has notes falling down on the keys that are to be played. You can adjust the tempo and practice your hands separately, so that you learn at your own pace. This MIDI-based software makes learning to play the piano accessible and fun for everyone, especially if you just want to learn that one tune!