The Laws of Thermodynamics show that a perpetual motion machine -- a machine that produces more energy than it takes in -- is impossible under the system of science and physics as understood today. Conservation of energy states that no matter or energy can be destroyed or created, just transferred/changed, and that the total amount of matter/energy in the universe is constant. For a machine to produce energy without being fed energy, or to produce more energy than it is fed, would violate these laws. Every single perpetual motion device ever attempted has failed to work under scrutiny and proper testing; many had batteries or gas engines inside them, giving the illusion of greater energy while actually expending far more. The only way this idea can work is to alter the statement; you cannot produce energy without expending energy, but you can build a passive system that collects energy from existing energy sources such as the Sun, wind, or geothermal vents. These passive systems will continue to work as long as their energy sources remain constant, but are not creating energy from nothing; they are simply changing energy from one form to another.