The questions and answers in the above exchanges are common occurrences in classroom discourse: requests by the teacher for causal explanations and efforts by the students to give them. To succeed in school, students need to be able to explain causally, and teachers need to be able to assess these explanations. Students’ causal explanations allow teachers to check understandings of how and why; thus, examining the development of this type of discourse has the potential to provide a framework for formative assessment that can promote learning. Researchers and educators working from a systemic functional linguistic perspective have provided a body of work on causal discourse in science, offering an excellent starting point for examining the development of causal explanations in that subject area. Much of the work that has been undertaken has generally focused on texts written by expert writers (e.g., Mohan et al., 2002 ; Veel, 1997), such as textbooks and encyclopedias.