Incandescent bulbs produce light by heating a thin filament wire until it reaches a temperature that causes it to glow. As a result, these bulbs expend most of their energy in the form of heat, not light. This heat-driven way of producing light causes this little bulb to burn out quickly, so even though an incandescent bulb may cost only about $2, it only has a lifespan of about eight months. Soon, this inefficient bulb will be a thing of the past. As of January, 2012, 100-watt bulbs will no longer be imported, manufactured or sold in the United States, and by 2014 all incandescent bulbs will be phased out.