http://en.wikipedia.org/wiki/Fatigue_(material) http://en.wikipedia.org/wiki/Fatigue_(material) The idea is: Turn it on, metal expands (thermal expansion) Turn

http://en.wikipedia.org/wiki/Fatigue_(material) http://en.wikipedia.org/wiki/Fatigue_(material) The idea is: Turn it on, metal expands (thermal expansion) Turn it off, metal contracts This is the repeatedly applied load, hence eventual material failure. Also, the hotter you run the bulb, the more tungsten evaporates.