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CAST IN STONE |
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CAST IN CONCRETE, CARVED IN STONE |
People expressing flexibility say that their ideas or rules are “not cast in concrete,” meaning they have not hardened into rigidity. You cast concrete in a mold by pouring it in and letting it set; so the expression can also be “not set in concrete.”
A similar expression is “not carved in stone” (like the Ten Commandments).
People frequently mix these two expressions up and say things like “It’s not cast in stone.” They may be influenced by the unrelated Christian saying, “Don’t cast [throw] the first stone.”