To select which song you would like to play, just insert one of the 4 song cartridges into a slot on top of the box. Embedded in each cartridge is a light filter with a different opacity. Inserting the cartridge triggers an LED to shine light through the filter, and a light sensor determines which song cartridge was inserted by reading the amount of light passing through the filter. Two different littleBits synth modules, Envelope and Delay, give you a chance to play with the audio single creating a more dynamic sound.
See it play here: https://www.youtube.com/watch?v=o97j4Lim8H8