I like the idea of knowing how to essentially grow your own pH testing solution. Many plants are pH reactive with a corresponding coloration. Not all plants are equal because the difference in the spectrum of colors which provides more visual clarity on the results is not the same. This is why cabbage is used versus hibiscus for example which only changes the color saturation. Either way, if you didn't know and wanted to learn how to make your own litmus tester here you go.
Other links:
http://www.braukaiser.com/wiki/index.php/An_Overview_of_pH 
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