Physics
On a clean glass plate a drop of water spreads to form a thin layer whereas a drop of mercury remains almost spherical because:

cohesion of mercury is greater than its adhesion with glass.
density of mercury is greater than that of water.
mercury is a metal.
cohesion of water is greater than its adhesion with glass.

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Physics
Planets do not twinkle because

They emit light of a constant intensity
They are nearer to earth and hence we receive a greater amount of light.
Their distance from the earth does not change with time
They are very far away from the earth resulting in decrease in intensity of light

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