【俟河之清】〖sì hé zh qng〗[wait till the Yellow River runs clear water; once in a blue moon; wait for the millennium]

俟奋韬光著诗典，

河出伏流中外传。

之明先见世界语，

清旷超俗大同寰。

等待黄河变清。比喻期望的事情不能实现。Wait for Yellow River become clear. metaphor expect thing cannot be fulfilled.