Question

Why are the taps left dripping in sub zero temperature during winter?

Answer 1

The dripping taps will always create some space for expansion.Thus, when water in the pipes freeze in sub-zero temperatures,due to its anomalous expansion,it expands.This expanding water always finds space for expansion as tap is dripping. Thus,water pipes do not burst.

Answer 2

It is done so to provide sufficient space for expansion of freezing water due to it's anomalous expansion. If this not done, the freezing water will exert large pressure on the tap and there are chances that they may burst.