So when someone describes a plan or course of action that we believe may be impossible to achieve, we tell him, "You're building castles in the air."
所以，当有人描述一个计划或行动时，我们觉得那是不可能实现的，我们会告诉他：“你的想法就如空中楼阁一样B+w.hC;I7PLrxnK~B。”
A similar idiom is "building castles in the sand."
类似的俗语还有“在沙滩上建城堡LSSRw@bq4F9(b。
We know that, on a beach, sand castles are inevitably and easily destroyed by the tide.
”我们知道，沙滩上的城堡不可避免且容易被海浪摧毁@HI.1GH8b0zIx.p(。

They are not built to last a long time.
这样的城堡维持不了多久[G;H+~Tk%esTq。
So when we say someone is "building castles in the sand," we mean they are creating something that won't endure very long.
所以，我们说有人是“在沙滩上建城堡”，其实就是指他们正在创造的东西持续不了多长时间B*AA(|,|98vRFv|tV&)。

译文来自可可英语