Litmus paper is a special strip of paper used in science to test whether a liquid is an acid or a base. When you dip it into a substance, the paper changes color: it turns red in acids (like lemon juice or vinegar) and blue in bases (like soapy water or baking soda mixed with water).

Scientists have used litmus paper for hundreds of years because it gives quick, reliable answers. The paper gets its color-changing ability from lichens, which are organisms that grow on rocks and trees. When treated with chemicals from these lichens, ordinary paper becomes a simple but powerful scientific tool.

Today, people use the phrase litmus test to mean any simple way to judge or measure something important. A teacher might say that understanding fractions is a litmus test for being ready for algebra. A coach might call free throws a litmus test of a basketball player's concentration. Just like the actual paper reveals hidden properties of liquids, a litmus test reveals the truth about a situation or skill.