if your talking about the incandescent... they use tungsten (middle part the goes horizontal) and use energy to burn it... tungsten is used because it can burn for a long time but yet still not break.... when the tungsten gets burned for a long time than it breaks.... tungsten uses the power source as energy to burn thats why it makes ur electric bill go up.. i dont really know how the other types of light sources burn out
the filiment gets to metal fateg and breaks...yes i know my spelling sucks
