Crookes Radiometer is device which generates mechanical energy from light. The instrument consists of a shaft with Blades (leaves). The blades are painted black at one side and silver at other side. When light is shown on the device the leaves rotates.
Please refer to http://en.wikipedia.org/wiki/Crookes_radiometer to know more about Crookes Radiometer.
According to the current scientific knowledge base, the explanation of Crookes Radiometer revolves around 'Thermal Transpiration'. However the explanation is very convoluted. Please refer http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html for details about the current explanation.
I have better explanation.
The main contention is about how temperature at both sides of the blade (leaf) causes rotation.
First consider what happens at the boundary between the blade and gas around it. The molecules in the gas hit and bounces at the blade. Consider now the temperature of the blade is higher than that of gas. Now heat will flow from blade to gas. How is heat flow from blade to gas? When the molecules hit the blade. When the molecules hit and bounce, it will bounce with faster speed than it was traveling before bouncing*. The increase in speed means increase in energy (the energy transferred from the blade to gas). The increase in speed is depends on the temperature difference between the blade and gas.
Similarly when the temperature of the blade is less than the gas, the molecules will bounce with a lower speed than it was traveling before touching the blade.
In steady state, temperature difference between gas and black side is higher than the temperature difference between the gas and silver side. Hence the molecules at black side bounces faster than the silver side.
As the molecules bounces faster at black side, black side will experience more pressure than silver (note that the pressure = change is momentum; momentum = mass * velocity). This pressure difference causes the rotation of the blade.
* How can a particle travel faster than it's initial speed? Well, actually the real story is not like that. According to quantum theory the particle when hit get absorbed in the surface of the blade and then it get emitted with a higher speed. However this is not important to us.