Counterfeit banknotes made by a truly clever forger will never be discovered, so once his work is recognized as a forgery, the forger is not a clever forger, and the true forger will never be caught.
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Xiao Zhang is a magician. His magic is generally not seen through by people. But occasionally, he is seen through once or twice, indicating that he is not a smart magician, because a smart magician will not be seen through.