litmus test

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The Word of the Day for September 29 is:

litmus test   \LIT-mus-TEST\   noun 
     : a test in which a single factor (as an attitude, event, 
or fact) is decisive

Example sentence:
     For Curtis, the litmus test of good barbecue ribs is 
whether or not they have that moist fall-off-the-bone quality.

Did you know?
     It was in the 14th century that scientists discovered that 
litmus, a mixture of colored organic compounds obtained from 
lichen, turns red in acid solutions and blue in alkaline 
solutions and, thus, can be used as an acid-base indicator. Six 
centuries later, people began using "litmus test" figuratively 
for any single factor that establishes the true character of 
something or causes it to be assigned to one category or another.