Physics
The force of friction between two surfaces will increase if:

Irregularities on both the surfaces are removed
Air gap is created between the two surfaces
The two surfaces are pressed harder
A layer of lubricant is kept between the two surfaces

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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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