Physics
Siphon will fail to work if:

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

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Physics
Ice is packed in saw dust because

Saw dust is poor conductor of heat.
Saw dust does not stick to the ice.
Saw dust is a good conductor of heat.
Saw dust will not get melted easily.

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