get blood out of a stone
To do something that is very hard to accomplish, especially when dealing with other people in some way. Good luck getting a group of toddlers to sit still—it's like getting blood out of a stone. Please, Dad will never give you money for something so frivolous. You can't get blood out of a stone. If you think I'll give you the information you're looking for, you're sorely mistaken! You can't get blood out of a stone!
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