The only time I've worked with litmus paper was in Sister Guillory's classes around 1969. We dipped one end of a slip of paper into a dish of liquid to determine its acidity. We tested liquids like lemon juice, vinegar, Clorox bleach. If the substance was acidic, the paper changed color to a reddish pink. If it was base (alkaline), it turned blue. There was a number scale - the darker the red, the higher the number reflecting the strength of the acid. The darker the blue, the more basic, earning a negative number. There were some neutral substances - I think I remember water as one of them - that were right in the middle, ranking 0. The paper turned a pale purple. A brief glance at internet resources suggests that current tests and scales may be different than what I recall us using back in the 1960s. The litmus test was easy to use and comprehend, a fun way to learn about acidity.