Fossil fuels are formed under earth’s surface from dead remains of plants and animals. It is a very slow process and it takes millions of years to be formed. If they will be consumed at a very faster rate then the rate at which they are formed, they soon will get exhausted. Our future generations will not be able to make use of them as it takes millions of years to be formed. So it is said that fossil fuels will last only for a few hundred years.