A litmus test is a simple way to determine something important about a situation, person, or idea. The phrase comes from chemistry, where scientists use a strip of special paper called litmus paper that changes color when dipped in a liquid: it turns red in acids and blue in bases. This quick color change reveals something essential about the substance.

Outside the laboratory, people use “litmus test” to describe any straightforward way of judging or evaluating something. A coach might say that how players handle losing is a litmus test of their character. A teacher might view how students treat a substitute teacher as a litmus test of their maturity and respect. If your parents say that keeping your room clean for a month is the litmus test for getting a pet, they mean it's the key indicator that shows whether you're responsible enough.

The term suggests a clear, revealing measure: just as litmus paper gives an immediate answer about acidity, a good litmus test quickly shows what you need to know. When someone proposes a litmus test for friendship, they might suggest something like “Do they stand up for you when others criticize you?” A single observation or question can reveal a larger truth about the whole situation.