On today’s episode, you’ll hear that prosecutors investigating the McDonald’s scam figured the whole thing out—but they wanted to let McDonald’s run one more competition so they could catch the criminals in the act. They wanted rock-solid proof that the whole thing was a fraud. “Rock-solid” means definite proof—proof that could not be questioned. In other contexts, you could say that two people have a rock-solid relationship: they have a very strong marriage. Or, you can say that a person has a rock-solid reputation. That person’s reputation is very strong and cannot be questioned.