Just hit green flag and watch and listen. It's pretty simple!
On my computer this demo bogs way down. (This happens both within the web app and with the stand alone Scatch application.) Probably the number of clones is too great. As a result the lines of falling "Happy Birthdays" overlap and the song slows way down. The song takes 23 seconds to play while the entire project is running, but if you go inside the project and just play the song by itself, it only takes 19 seconds for the song to play. (The song is contained in the background. It's not a recorded song, it's a coded song.)
