Here's why:

* Litmus paper is a type of indicator paper that changes color depending on the acidity or alkalinity (pH) of a solution.

* Acids have a pH less than 7.

* Blue litmus paper is designed to detect acids. When it comes into contact with an acidic solution, it will turn red.

* Red litmus paper, on the other hand, is designed to detect bases (alkalines), which have a pH greater than 7. It will turn blue in the presence of a base.