Here's why:

* Litmus paper is used to test for acidity and alkalinity (basicity).

* Red litmus paper is already red in acidic solutions. It will only change color if it is dipped in a basic (alkaline) solution, turning blue.

* Blue litmus paper is used to test for acids. It will turn red when dipped in an acidic solution.

So, to summarize:

* Red litmus paper stays red in acids.

* Red litmus paper turns blue in bases.

* Blue litmus paper turns red in acids.

* Blue litmus paper stays blue in bases.