A few examples of heat sources are the sun, friction, chemical reactions and the earth. The sun is a natural heat source that is renewable and that can be converted into electricity.
Life on earth depends on heat from the sun. This heat is generated from nuclear reactions that take place within it. A fire can be generated from the process of rubbing sticks together, which is one example of friction. The heat within the earth is known as geothermal energy. When this heat is close enough to the surface of the earth, it can be drilled, extracted and used to heat buildings.