A simple pull string mechanism powered via rubber band. The spool posts are a bit long so someone could put a gear on it and use this to power a clockwork contraption or pull string toy. (If you don't need the extra lenth just clip them to size) Or you could use it as a retractable key leash {or lighter leash combined with my bic lighter holster) Also makes a great powered wheel. Just omit the string and wedge the rubber band between the two shells after pulling it through the center spool. Attach the wheel by the posts of the spool and the shell will turn freely around the spool. A good use for this would be maybe a pull car.