Litmus paper is a type of pH indicator used to test the acidity or alkalinity of a solution. It is made from paper that has been treated with natural dyes derived from lichens. When dipped into a liquid, blue litmus paper turns red in acidic solutions (pH less than 7), while red litmus paper turns blue in alkaline solutions (pH greater than 7). This simple tool is commonly used in laboratories, classrooms, and even at home for quick pH testing. It provides a visual way to determine the nature of a solution without needing complex equipment.