A river-monitoring group received reports that fish had disappeared near a factory. One resident had seen no fish on a single cloudy afternoon. Another had counted fish weekly at the same three locations for six months. The second record offered a stronger basis for describing a change, but it did not by itself identify the factory as the cause. Water temperature, stream flow, and activity farther upstream could also matter. Investigators compared measurements above and below the outlet and examined when the reported decline began. Each new observation answered a more specific question. Evidence that something changed, evidence that it changed in a particular location, and evidence explaining why it changed are not interchangeable. A careful investigation advances by matching the strength and kind of its conclusion to the question its observations can actually answer.