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

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

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Physics
The wall of a dam is broader at the base

To withstand pressure that increases in a horizontal plane
To withstand pressure that increases with depth
Because streamlining is required
To withstand pressure that is increased with atmospheric pressure

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