Castles are usually enormous structures requiring strong foundations and the use of heavy construction materials such as stone. If the foundations are not strong enough, the castle could collapse. The idea of building a castle in the air-with no foundation at all-is, of course, ridiculous. That is why this idiom is used to describe ideas that are not realistic, as if they are dreams or fantasies. 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."
类似的一个成语是“在沙滩上建城堡。”我们知道,沙滩上的城堡不可避免地而且很容易被海浪摧毁,不可能持久。所以,我们说有人是“在沙滩上建城堡”,意思就是他们正在创造的东西持续不了多长时间。
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