Materials
Cardboard box
Two 50-70 cm pieces of strong, coloured thread
Two pieces of compact foam polystyrene cut to fit the opposite walls of the box, both with a hole in the centre
A plastic tube (e.g. from a pen) and an axis to be inserted inside the plastic tube (this latter is as long as the opposite sides of the box and permits the external tube to slide and to rotate freely; the tube is shorter then the internal axis so that it is free to rotate once it is mounted in the box)
A spool that can be fixed on the tube with the larger diameter
One or two discs adaptable to the rotating tube, to control the rolling-up of the thread
Two nuts.
Method