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
Lightning is produced when

Clouds strike against impurities in air and the friction burns up these impurities
Water vapour produces electricity in the clouds
Similar charges of electricity rush towards each other and then get repelled
Strong opposite charges in different clouds break down the resistance offered by the intervening air

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