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.Print all three parts. You need supports for the spool. You may be able to get away with printing the shells sans-support providing it was a relatively small print. Coil a bit of string around the spool (Use the hole in the post to tie the string through.) Push the string though the inner shell and then the outer shell. Pull a rubber band through the other hole in the center spool and out the two sets of holes in the sides of the inner and outer shells (in that order). Cut a small piece of filament to the length of the diagonals of the square hole and loop the rubber band around it. Pull the string a bit and tie it through something like a bolt to prevent the string from fully retracting. This will pull the filament pieces into place and keep them there. This is pretty scalable so feel free to resize it for projects that need more power or longer string.