Perpetual motion violates either the first law of thermodynamics, the second law of thermodynamics, or both. The first law of thermodynamics is essentially a statement of conservation of energy. The second law can be phrased in several different ways, the most intuitive of which is that heat flows spontaneously from hotter to colder places; the most well known statement is that entropy tends to increase, or at the least stay the same; another statement is that no heat engine (an engine which produces work while moving heat between two separate places) can be more efficient than a Carnot heat engine. As a special case of this, any machine operating in a closed cycle cannot only transform thermal energy to work in a region of constant temperature.
Machines which are claimed not to violate either of the two laws of thermodynamics but rather to generate energy from unconventional sources are sometimes referred to as perpetual motion machines, although they are generally considered not to meet the standard criteria for the name. By way of example, it is possible to design a clock or other low-power machine, such as Cox's timepiece, which runs on the differences in barometric pressure or temperature between night and day. Such a machine has a source of energy, albeit one from which it is impractical to produce power in quantity.
