Physics
Skating on ice is possible because

Ice is cold
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
The surface of ice is smooth

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Physics
Specific gravity is defined as the ratio of

Density of the substance to the density of water at 4°C
Density of the substance to the density of water
Density of water at 4°C to the density of the substance
Density of the substance to the density of water at 0°C

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