Energy has many different forms like gravitational, kinetic, electrical, chemical energy and as we know from Albert Einstein, even mass. According to the first law of thermodynamics the total amount of energy remains constant and can only be converted from one form into another. We are familiar with processes like the acceleration of a stone falling to earth (conversion of gravitational into kinetic energy), or the burning of gasoline to drive our car (conversion of chemical into kinetic energy). But what about "water" that drives a swinging object or generating electricity from falling water drops? Can you imagine the direct conversion of light into rotational movement? Or the conversion of heat into kinetic energy in an engine mainly built of a special metal wire?