Physics
Siphon will fail to work if:

The level of the liquid in the two vessels are at the same height.
Both its limbs are of unequal length.
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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Physics
A stone floor feels cold to the bare feet but a carpet on the same floor feels warm. This is because

The stone conveys the heat away from the feet more rapidly than the carpet
The floor and carpet are at different temperatures
The carpet is a better conductor of heat than the stone
The human body maintains a constant temperature

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