Acid test
Meaning: the most crucial or important test of worth, the true test of the value of something
Example: It looks good, but will people buy it? That's the acid test. The acid test for laundry soap is not how well it cleans in hot water, but how well it cleans in cold water.
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Example: It looks good, but will people buy it? That's the acid test. The acid test for laundry soap is not how well it cleans in hot water, but how well it cleans in cold water.