All life on earth is sustained by energy from the sun. Plants and animals can store energy. Some of this energy remains with them when they die. It is the remains of the stored-up energy of ancient animals and plants that make up the fossil fuels that power our cars and factories today.But fossil fuels are non-renewable energy sources and will one day run-out and because burning them generates greenhouse gases, relying on fossil fuels for energy generation is unsustainable. Hence the need to find more renewable, sustainable ways of generating energy.
