This study examines question asking about "causal" relations when students read scientific texts. We examine the influence of the length of causal chains and the knowledge of readers. Students from grades 8 and 12 read two short paragraphs that described natural phenomena. Length of the causal chain linking cause and effect in two key sentences was manipulated. The students were instructed to ask on anything that they did not understand. The results showed, in the first place, that students ask a majority of causal antecedent questions. Second, a longer causal chain resulted in less causal questions asked by the 12th grade students, but did not reduce the number of causal questions asked by 8th grade students. Third, there was no relation between comprehensibility ratings given to the causal relations and causal question asking. According to this, an explicit comprehensibility score that taps on the metacognitive awareness of students may not be a good indicator of the level of understanding.