Physics
stars which appear single to the naked eye but are double when seen through a telescope are

binaries
novas and supernovas
quasars
asteroids

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Physics
Siphon will fail to work if:

Both its limbs are of unequal length.
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.

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