The greatest problem that faces the world today is global warming. It is more apparent here in India than anywhere else, especially Rajasthan, where temperatures over the last few years have risen from 32°C to 36°C to last summer's 42°C to 46°C. The principle forms of renewable energy suitable for places that lie in the tropics are wind and solar energy. The solar panels seen on the roof tops are usually for producing hot water and should not be confused with those used to produce electricity, which are photovoltaic panels. They are made of 2 thin plates of silicon containing slight impurities, which, when exposed to sunlight, experience a stimulation of electrons. If positive and negative terminals connected by a wire are added, as in a battery, the electrons will flow around the wire, producing electricity.