The energy we get today from the fossil fuels like coal, is in reality sun's energy, trapped in them millions of years ago. Plants make their food and grow because they use solar energy for photosynthesis. Millions of years ago, huge forests got buried in the earth's crust and under great pressure and temperature, they were converted into coal or oil. Hence coal and oil are called fossil fuels. Solar energy is the great source for production of vegetation which serves as food and fuel for us. However, nowadays, we have learnt to trap solar energy for various purposes. Solar energy can be used directly to give us hot water during winter, or run a refrigerator. It can be used for heating rooms in colder regions. It can also be used, with the help of a "photocell" to produce electricity for driving vehicles and illumination of streets. In a desert like Rajasthan, the earth's surface receives solar energy at the rate of 200 watts per square metre per hour. Since this is an unfailing source of energy, it would be a great advantage to develop cheap and efficient photocells or photovoltaic devices to harness solar energy. A feature of using solar energy is that it can be generated where needed. Solar cookers are being used in many homes to cook food. Photocells are also becoming common, though their efficiency has to be improved and their price brought down.