Theory: A solid object like a rod has resonant frequencies just like those in a tube open at both ends. In both cases, the ends are anti-nodes. When the rod is held at a node of a harmonic and that harmonic is excited, it will ring clearly for several seconds.
Description: The nodes of the fundamental and the second and third harmonic are labelled with tape on the rod. To excite the fundamental, grip the rod in the center with one hand and slide the rosined fingers of your other hand firmly along the rod. It should sing.
This rod is almost too flexible for this demo, but it sure can sing. To excite the second harmonic, grip the rod 1/4 of the way along the rod. You won't be able to support the rod like this, so rest it on a support (like the edge of a bench) at the 3/4 mark (at another node, that is). Again, it should sing when you slide your rosined fingers firmly along the rod. It's easy to hear the difference between holding the rod at a node of a harmonic and holding it at some random spot on the rod, too; the rod will only sing when you hold it at a node. To get the third harmonic, grip the rod at the 1/6 mark, and support it at the 5/6 mark.
Setup time: 5 minutes
References: (available in the demo room)