Energy is movement or the possibility of creating movement. It exists in potential (stored) and kinetic (used) forms, and can be converted from potential to kinetic. Movement states can be ordered (mechanical energy) or disordered (thermal energy), with temperature perceived as a level of disordered energy. The major tendency is for energy to move from order to disorder (entropy). Human activities depend on various forms and sources of energy, with energy demands increasing with economic development. The world's power consumption is about 12 trillion watts a year, with 85% coming from fossil fuels. Sources of energy include chemical (fossil fuels combustion), nuclear (uranium fission of atoms), muscular (oxidization), geothermal (conversion), fusion (fusion of hydrogen), and gravity (tidal).