Physics
Planets do not twinkle because

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

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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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