anyone (at least those who play the piano) should have played or heard someone playing this mysterious and anonymous tune before. usually it's played by four hands, and it seems that nobody knows how to end the song, but there IS actually an ending! i remember jacinta saying she saw tom hanks playing it by tapping his feet on a giant walking piano in a movie. just now i searched on youtube and found the answer - the 1988 tom hanks movie is called "Big", and the song is called "heart and soul" written by american composer Hoagy Carmichael in 1938.
this video features some wonderflly sophisticated improvisations of the simple tune.