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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Physics
Planets are

luminous heavenly bodies revolving around a star
luminous heavenly bodies that do not twinkle
non-luminous heavenly bodies
luminous heavenly bodies that twinkle

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