Have you ever found it difficult to read something due to your lack of knowledge on the topic? We investigated this phenomenon with a sample of 3,534 high school students who took a background knowledge test before working on reading comprehension tasks on the topic of ecology. Broken-line regression revealed a knowledge threshold such that below the threshold the relationship between comprehension and knowledge was weak (beta=0.18), but above the threshold a strong and positive relation emerged (beta=0.81). Further analyses indicated that certain topically relevant words (e.g. ecosystem, habitat) were more important to know than others when predicting the threshold, and these key words could be identified using natural language processing techniques. Collectively, these results may help identify who is likely to have a problem comprehending information on a specific topic, and to some extent, what knowledge is likely required to comprehend information on that topic. [This paper was published in "Psychological Science."]