Coal- It is a hard stone like black coloured solid fossil fuel. It is used for cooking food. Steam based railway engine used coal to produce steam for running pistons. In thermal power plants, coal is used to generate electricity. Different industries use coal as a fuel. Petroleum (Petra-Rock; oleum=oil)-Petrol and diesel like oils are obtained from petroleum. Petroleum is formed by the carbonization of fossils of animals and tiny plants like planktons. It is found in between the layers of impervious rocks inside the earth. It is found at a depth of few kilometers to few hundred kilometers. Petroleum is a dark oily liquid. It has unpleasant odour. It is a mixture of different hydrocarbons.