Physics
Siphon will fail to work if:

The densities of the liquid in the two vessels are equal.
Both its limbs are of unequal length.
The level of the liquid in the two vessels are at the same height.
The temperature of the liquids in the two vessels are the same.

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Physics
Skating on ice is possible because

The surface of ice is smooth
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

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