 Water itself is highly stable—it was one of the classical elements and contains very strong chemical bonds.Its enthalpy of formation is negative (-68.3 kcal/mol or -285.8 kJ/mol), meaning that energy is required to break those stable bonds, to separate water into its elements, and there are no other compounds of hydrogen and oxygen with more negative enthalpies of formation, meaning that no energy can be released in this manner either.Most proposed water-fuelled cars rely on some form of electrolysis to separate water into hydrogen and oxygen and then recombine them to release energy; however, because the energy required to separate the elements will always be at least as great as the useful energy released, this cannot be used to produce net energy.