In a Chinese farmers market, stooges handed over either clean or dirty notes, then asked for 500g of an extra vegetable.

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So does black market, or the supposedly clearly distinct clean and dirty money.

NPR: Tracking the World's Black Market: 'Illicit'

Under the center consoles you can tell if the carpet appears genuinely clean because it is or if it was cleaned from being dirty.

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Or can a down and dirty approach sometimes help you clean up?

BBC: The bottom line on being messy

You can also shake the device to clean the text in case it ends up getting "dirty, " or misaligned.

ENGADGET: E Ink reference phone and flexible display hands-ons (video)

More fascinating, less dirty - Dyson now engineers motors in a room so clean no pollutants, pollen or dirt can damage the finely tuned technology being made by the highly calibrated production line.

BBC: Technology