Imagine a music box that plays “Row row row your boat.” It has a cylinder with raised dots on it that pluck the teeth of a comb. This music box has three combs, spaced four beats apart (you’ll see what I mean). The leading comb is fixed in place, but the other two combs can be pulled away from the cylinder
so they don’t play.
Start with only the first comb in contact with the cylinder. Wind it up and it starts playing. It goes “Row row row your boat.”
At that moment push the second comb in. That comb starts at the beginning of the tune with “Row row row your boat” while the first comb plays “gently down the stream.”
Now push in the third comb. The third comb starts in with “Row row row your boat” while the second comb plays “gently down the stream” and the first comb plays “merrily merrily merrily merrliy.”
Will also work with such classics as “Frere Jacques”, “Three Blind Mice” and many more!
(Thanks to lostdog for making me think of music boxes.)