Overtone singing is an amazing thing where by changing the shape of your mouth can create separate pitches in the overtones, even when the fundamental pitch remains the same.

This first video does a somewhat better job of explaining exactly what overtone singing is and how you can practice it.

This next video shows how you can use the overtones produced to create an actual melody. Listen carefully to hear “Amazing Grace” in the overtones. It might take you a few tries.

Finally this is an example of a piece of composed music that uses overtones for a neat effect.