Relying on multiple documents to answer questions is becoming common for both academic and personal inquiry tasks. These tasks often require students to explain phenomena by taking various causal factors that are mentioned separately in different documents and integrating them into a coherent multi-causal explanation of some phenomena. However, inquiry questions may not make this requirement explicit and may instead simply ask students to explain why the phenomenon occurs. This paper explores an "Activity Model Hypothesis" that posits students lack knowledge that their explanation should be multi-causal and how to engage in the kind of thinking needed to construct such an explanation. This experiment, conducted on a sample of eigth grade students, manipulated whether students received a short 10-min lesson on the nature of scientific explanations and multi-causal reasoning. Students who received this causal chain lesson wrote essays that were more causally complex and integrated, and subsequently performed better on an inference verification test, than students who did not receive the lesson. These results point to relatively simple changes to instructions that can provide the support students need for successful multiple-document comprehension.