This is so awesome.

The CD insert is actually a circuit board that lists all the tracks and artist information – cool enough, but there’s more. This circuit board has a headphone jack on it and a light sensor, and it acts as a fully-functioning theremin! To work it, you press a button with one hand, and wave your other hand in front of the light sense. It’s expensive ($50), but he also has a mini-edition for significantly less.