blood out of a stone

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. A: "Why don't you ask your dad for the money?" B: "Because asking my dad to lend me money is like trying to squeeze blood out of a stone." A: "Good luck getting enough Republicans to support this bill." B: "Yeah, I know it'll be like getting blood out of a stone."