The purpose of this investigation was to determine the impact of students' background knowledge and how they utilized "don't know" affordances to comprehend and learn from text. In two studies, over 8,000 middle and high school students interacted with a content-area learning environment in which they answered a series of background knowledge questions before they completed a unit on the same topic. Students were given the opportunity to indicate they "did not know" the answers to the knowledge questions. Higher knowledge was related to higher understanding, and the use of the "don't know" option further explained variability in students' understanding of the sources beyond background knowledge. When responding to knowledge questions, students who selected incorrect options before the task understood less and were less likely to learn content when given the opportunity compared to students who indicated they did not know. Thus, low knowledge students were still able to comprehend and learn as long as they acknowledged they lacked background knowledge. One's comprehension and learning can be facilitated or impaired, depending upon the veracity of their knowledge, and whether students choose to acknowledge their lack of background knowledge. Implications of this work are discussed in terms of learning and instruction. [This paper was published in "Reading Psychology" v40 n7 p638-677 Sep 2019 (EJ1230363).]