【成语来源】 

castles in the air
空中楼阁

A rich man asked an architect to build a three-story house for him.
有一个富人让一位建筑师为他建造一个三层楼的房子。

When the first story was finished, the rich man said to the architect: "I want only the third story; not the first and second stories."
当第一层建造好的时候,富人对建筑师说:“我只想要第三层楼,不要第一、第二层。”

The architect asked, "But without the first and second stories, how can I build the third story?"
建筑师说:“但没有第一、第二层,何来第三层呢?”

Shaking his head, he packed up his things, and left.
建筑师于是就摇了摇头,收拾东西离开了。

This idiom indicates an unrealistic or impractical plan or theory.
这个成语现在比喻不现实或者不可行的计划或者理论等。

【文化链接】

英文中“空中楼阁”可以说成“castles in the air”,基本就是直译了,所以不难理解哦~
英文解释是“plans or hopes that have very little chance of happening”,也就是指“发生机会微乎其微的计划或者希望”。
我们来一起看几个例句吧~

She tells me she's planned out her whole career, but as far as I can see it's all just castles in the air.
她告诉我她已经有了整个职业计划,但是就我看来其实只是“空中楼阁”而已。

Before you start building castles in the air, just think how much all this is likely to cost.
在你开始建立“空中楼阁”之前,不如先想想你需要付出多少代价。