The Crookes Radiometer is one of those historical curiosities that even today has scientists hotly debating back and forth about how it actually works.
The short version of the description is that the device contains a spinning series of fan blades, the inside of the glass bulb is almost a vacuum and when you place it in front of a light source, the fan blades begin to spin. The rate of spin increases with the power of the light source, though exactly how this all works is still not entirely clear.
The Radiometer pictured here is 8.5 inches high and is designed to adorn a desk, windowsill or mantlepiece. You can pick one up for €45.00 from Concrete Matter, then sit back and enjoy the show.