Litmus is a natural dye extracted from certain lichens that changes color when it touches acids or bases, making it useful for quick chemical tests. Scientists soak paper strips in litmus to create litmus paper: blue litmus paper turns red when dipped in an acid (like lemon juice or vinegar), while red litmus paper turns blue in a base (like baking soda dissolved in water). This simple color change lets you quickly identify whether an unknown liquid is acidic or basic.

The word has also come to mean any simple test that quickly reveals something important about a situation. Politicians talk about litmus test issues, meaning questions that immediately show where someone stands. If your teacher announces a pop quiz and you think, “This will be the litmus test for whether I've been keeping up with the reading,” you're using the word the same way scientists do: to describe something that clearly shows whether a condition is present or absent.

The chemical version was discovered in the 14th century and remains one of the simplest, cheapest ways to test for acids and bases in a laboratory or classroom. When you use litmus paper in science class, you're using the same basic tool that chemists have relied on for hundreds of years.