Here's why:

* Litmus paper is used to test the acidity or alkalinity (pH) of a solution.

* Red litmus paper turns blue in the presence of a base (alkaline solution).

* Blue litmus paper turns red in the presence of an acid.

* Salt is neutral meaning it is neither acidic nor basic. It has a pH of 7.

Therefore, salt won't react with litmus paper and won't change its color.