Fossil fuels are energy resources that come from the remains of plants and animals. These remains are millions of years old. Coal, oil and gas are called "fossil fuels" because they have been formed from the organic remains of prehistoric plants and animals. Fossil fuels are currently the most economically available source of power for both personal and commercial uses. In 2005, more than 3/4 of total world energy consumption was through the use of fossil fuels. Long thought to be inexhaustible, fossil fuels have been used extensively since the Industrial Revolution. However, many believe that the world is using fossil fuels at an unsustainable rate. Fossil fuels have many hydrocarbons, each with numerous bonds. When they undergo combustion, they release a great deal of heat. This is the main reason why natural gas and heating oils are used extensively in the world today.