Litmus is a substance, made of lichen-based dyes, that is absorbed in paper and used to test acidity. Blue litmus turns red when exposed to acidic materials and red litmus turns blue when exposed to nonacidic materials. This is the origin of litmus test in its figurative sense—i.e., a test that draws broad conclusions based on a single factor. By extension, it’s also used to refer to a small result that is useful for predicting a larger result.
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