Physics
After long periods of use, a grey spot develops on the inside of a bulb. This is because

The heat of the bulb scorches the glass at the top
Dust inside the bulb condenses on the top
The tungsten filament evaporates and collects there
Glass undergoes a change due to the heat

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Physics
Absolute Zero is defined as

At which the volume becomes zero
At which water boils at 298K
The temperature at which all molecular motion ceases
At which liquid Helium boils

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