Incandescent light bulbs have a filament made of tungsten. The filament is heated to a white-hot temperature by the electrical current passing through it. The filament is so hot that the tungsten slowly evaporates off the filament and plates the inside of the glass envelope (bulb). The evaporation of metal causes the filament to get thinner over time. Repeated heating and cooling from turning the lamp on and off causes the filament to become brittle. A large surge current flows through a cold filament when the bulb is first turned on (the resistance of tungsten increases with temperature reducing the current as the bulb heats.) The surge passing through a brittle, thin filament can create a hot spot that completely melts the filament causing the light to burn out. This is why light bulbs tend to burn out when being turned on.