Although natural processes continually form fossil fuels, such fuels are generally classified as non-renewable resources because they take millions of years to form and the known viable reserves are being depleted much faster than new ones are being made. ... The use of fossil fuels raises serious environmental concerns.
Explanation: Once used, can't be renewed. For example, gasoline. Once put into a gas tank of a car, and you drive that car to empty, better hope there's a gas station near by!
