The drinking bird may also be considered to be an entropy engine driven by the difference of the entropy of liquid water and the entropy of water vapor dispersed in air, that is, the sum of the entropy of evaporation of pure water plus the entropy of dilution of water vapor in air. The evaporation of water is an endothermic process requiring the input of thermal energy or a positive enthalpy flow from the environment. Since a spontaneous process requires a negative change in Gibbs free energy, the positive enthalpy has to be overcome by the large entropy increase.