This simple (but very cool) switch uses Bare Paint (diluted with water 1:1) a syringe and two electrodes placed close together. As the material drips out of the syringe, one drop at a time, the drop hits the electrodes and closes the circuit. 

Adding a lot of syringes and connecting them to an Arudino gave a bank of switches.

Each switch was mapped to MIDI note, creating a cool musical interface!