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A light bulb consists of a thin filament, often made of tungsten, enclosed in a glass bulb filled with inert gas. When an electric current passes through the filament, it heats up and emits light as a result.

As the filament heats up due to the electrical current passing through it, its resistance increases. This is due to the increase in the temperature resulting in an increased agitation of the atoms and electrons in the filament. The increased resistance means that more energy is converted into heat, making the filament even hotter, and ultimately resulting in the filament glowing and emitting light.

When the filament heats up, it undergoes thermal expansion. This means that the material expands as its temperature increases. When the material cools down again, it contracts. This expansion and contraction can eventually lead to the material becoming weakened or brittle, particularly at the points where the filament is attached to its supports.

When a light bulb is turned on, there is a sudden surge of electrical current passing through the filament that rapidly increases its temperature. This results in a sharp increase in electrical resistance and a corresponding increase in temperature, causing rapid thermal expansion.

The rapid thermal expansion that occurs at the turn-on moment causes stress on the already weakened or brittle points of the filament. This additional stress can cause the filament to break, consequently resulting in the light bulb burning out. Since this process is more likely to occur when the filament is rapidly heated during the turn-on stage, light bulbs typically burn out just as they are turned on rather than while they are lit. In conclusion, the reason why light bulbs typically burn out just as they are turned on, rather than while they are lit, is due to the combination of increased electrical resistance, rapid thermal expansion, and the stress on the weakened or brittle points of the filament at the turn-on stage.