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."A similar idiom is "building castles in the sand."We know that, on a beach, sand castles are inevitably and easily destroyed by the tide.They are not built to last a long time.So when we say someone is "building castles in the sand," we mean they are creating something that won't endure very long.
