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

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

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Physics
Skating on ice is possible because

The pressure on the ice due to skate melts the ice by lowering its melting point forming a thin film of water
The pressure on the ice due to skate raises its melting point
Ice is cold
The surface of ice is smooth

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