Physics
Red light is used for signals because it has

High intensity
High frequency
Long wavelength
Low refraction in the medium

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Physics
Lightning is produced when

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

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Physics
Siphon will fail to work if:

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

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