Nǐ zhème kuài jiù xiǎng míngbai le, guǒrán rúzǐ-kějiào a!
You figured that out so fast. You're a promising pupil after all!
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In the 史记, an old man drops his shoe off a bridge and orders the young 张良 to fetch it and put it on his foot. Zhang Liang does, and the old man, later known as 黄石公, says 孺子可教矣, and after testing him further hands him a book of strategy.
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Praise from an elder or teacher to someone younger. Between friends it is playful, and said to a superior it sounds condescending.
