Department of Physics and Astronomy

With the sphere expanded, spin it on the floor. As it slows down, the sphere collapses... which will cause it to spin faster!

An updated version shows a better display, but does come with some additional notes.  First, the plastic has a lot of hignes and slots so the string can easily get snagged and entangled.  Also, while the hanger has fairly low friction, it is by no means frictionless!  In general, a slower starting spin is better since the increase in speed is more dramatic.  Like many demos, "gentle" and "careful" are good words to keep in mind.