Oil, natural gas and coal are called fossil fuels as they are derived from the remains of the living things, such as dead animals and plant parts. These remains were deposited under the sea, where they were covered by layers upon layers of sediments. These sediments were finally solidified into rocks over a period of millions of years. The animals and plants remains ultimately changed into these fuels with high temperature and pressure. Coal is a solid form while petroleum is a liquid form and natural gas is the gas form of the fossil fuels. Therefore, the reserves of these fuels are limited and they are called the non-renewable resources as they cannot be replaced once they have been used up. Usually, natural gas is also found wherever the petroleum is extracted. The reserves that are known today and are possible to exploit, are called the proved reserves. It is estimated that the proved reserves of petroleum in the world will be finished by the year 2045 AD. Therefore, it is very important to use these resources carefully and find other resources of energy such as solar energy, which is unlikely to finish.