I don't actually need an excuse to post a Theremin video

... but if I did, I would point out that a clip of the instrument followed by a quick discussion is a great way of opening a lesson on the Cartesian coordinate system.

In case you've forgotten:

The theremin is distinguished among musical instruments in that it is played without physical contact. The thereminist stands in front of the instrument and moves his or her hands in the proximity of two metal antennae. The distance from one antenna determines frequency (pitch), and the distance from the other controls amplitude (volume). Higher notes are played by moving the hand closer to the pitch antenna. Louder notes are played by moving the hand away from the volume antenna. Most frequently, the right hand controls the pitch and the left controls the volume, although some performers reverse this arrangement. Some low-cost theremins use a conventional, knob operated volume control and have only the pitch antenna. While commonly called antennae, they are not used for receiving or broadcasting radio waves, but act as plates of capacitors.

Plenty of choices for the video (just Google 'Theremin'). This one ought to go over well.