Biomass is energy derived from plant or plant products. Plants are producers and are capable of converting solar energy into chemical energy. When combusted, carbon dioxide is released, which adds to greenhouse gases which can lead to global warming. Wood is a traditional biomass fuel which has been used by humans for thousands of years. Other examples include dung and garbage which can be combusted to release energy. These are used to produce methane gas which is an excellent fuel source. Corn and other plants can be harvested to produce ethanol-- a fuel that can power vehicles. Biomass works on the principle that the energy stored in something that was once alive can be released again by the process of combustion.