Today we go back to school . We will talk about two terms from science class . Both are tests . But don't worry . You will not be tested at the end of the program . The first term is “litmus test .” The second is “acid test .” In science , we use both to determine the chemical make-up of something . In conversation , we use them to determine other things . One identifies where something belongs or whether it is acceptable . The other determines if something is real or good . But before we tell you which is which , let's talk about their meanings . First , litmus test . Now , you may remember doing experiments with litmus paper in a high school science class . Litmus paper is used to identify whether a solution is an acid or a base . The Merriam-Webster website says scientists have long known that litmus paper turns red in acidic solutions and blue in alkaline , or base , solutions . In fact , scientists have been performing tests with litmus paper since the 14th century . Litmus itself comes from lichen , a small plant that grows on rocks and trees . Today many English speakers use the term “litmus test ” in conversations . To them , it means something you use to judge whether someone or something is acceptable . So , a litmus test is a kind of sign or indicator . It helps you identify in which category something belongs or in which direction something will go . It helps you group things together . Here's an example . A friend of mine has a single litmus test for dating -- her dog . Her dog must like the man and he must like the dog . After several failed relationships , she found this litmus test to be the one sure indicator of a good fit . Our second scientific expression is “acid test .” Such a test proves the value , quality , or truth of something . An acid test can take many forms . It can be an important job or a difficult situation . It shows if someone or something is good enough to succeed . For example , beating the national champions was the acid test for the basketball team . It showed the team's players they were better than the champs , even for only one game . An acid test can also find out whether something is as good as people say it is , whether it works , or whether it is true . For example , a restaurant may look nice . But will people eat the food ? That is the acid test . An acid test also can test the value of something . For example , I wanted to take an online writing program , but I wasn't sure if it was the worth the money . Then I found out that 80 percent of the people who took the class got a book deal . That was enough of an acid test for me ! The term acid test also comes from an experiment – one involving gold . Some people say it dates back to the use of nitric acid to determine the gold content of jewelry . So , those are the two tests – litmus and acid . But don't mix them up . They are different . A litmus test determines how to categorize something ; in other words , where does it fit ? An acid test determines if something is real , valid , or high quality . Let's go back to our dog example . When choosing a partner , the woman's litmus test is her dog . But this does not really prove if he is a good choice . So , it is not an acid test . The man never said he would be the perfect partner for her . So , his validity is not being tested . And he might be a perfectly good partner for someone else -- someone who doesn't like dogs .