Feedboxes are sound toys that react to incoming sound in simple way, yet they behaviour may become quite complex.
Rhythm box plays semi-random beat, which complexity depend on the volume of incoming audio signal.
String box plays plucked or strummed chords (generated with Karplus-Strong synthesis) based on short, percussive sounds.
The boxes could be used together with any instrument, or they could create a feedback loop by listening and responding to each other, creating ever-changing rhythmic structures.
Feedboxes use PdDroidParty by Chris Mccormick (based on libpd library) to run Pure Data patches on Android devices.