Tearing a paper involves overcoming the cohesive force between the cellulose fibres (of which paper is made). In the case of dry paper this force is high and hence tearing it is not very easy. However, the cohesive force that is of electrostatic origin becomes weakened in the presence of water.
This is akin to the way table salt (sodium chloride)
dissolves in water due the weakening of the electrostatic attraction between
the positively and negatively charged ions. In the case of paper, the effect
becomes easily perceptible as paper is hydrophilic and absorbs water.
Once dipped in water, the water molecules can easily flow
into the spaces between the fibres, weakening the cohesive force between them
and making them susceptible to easy tearing. Courtesy : The Hindu
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