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When you hear someone suggest an unrealistic solution to a problem, do you say, "That sounds like a castle in the sand" or "It's a painting in the sky"?
I've heard a Japanese economist use these idioms in an interview session on TV. However, I have never heard them used in real English communication ,and they are not in my dictionaries.
Do you think they are popular expressions?
This comes from the Bible - in a parable from the Gospel of St Matthew, and means that the basis of something is uncertain. Your source has the sayings slightly confused - the other saying is "Castles in the air" - impossible dreams.
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Castle in the sand 
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