  The standard demon is supposed to be able to create a temperature difference in a gas, without expending work. The gas is in a container, divided in two by an insulated partition, but there is a hole in the partition just large enough for a single molecule to pass through. The gas has been allowed to equilibrate at some well defined temperature, and as a result the average kinetic energy of each molecule is (3/2)kT (we ignore internal degrees of freedom and assume the gas is monatomic), where T is the absolute (Kelvin) temperature scale and k is Boltzmann's constant.