Here's why:

* Litmus paper is a type of pH indicator. It's made from a dye extracted from lichens that changes color depending on the acidity or alkalinity (basicity) of a solution.

* Acids have a pH lower than 7.

* Blue litmus paper is designed to detect acids. The dye in blue litmus paper is a weak base. When it comes in contact with an acid, the acid neutralizes the dye, causing it to change color from blue to red.

Let me know if you have any other questions about acids or litmus paper!