Physics
The sky appears blue because of

Scattering of light
Emision of blue wavelength by the sun
Atmospheric water vapour
Reflection on sea water

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Physics
Siphon will fail to work if:

The temperature of the liquids in the two vessels are the same.
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.

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