This study investigated third graders' use and variation of linguistic resources when writing a science explanation. Using systemic functional linguistics as a framework, we purposefully selected and analyzed writing samples of students with high and low scores to explore how the students' use of language features (i.e., lexicogrammatical resources) reflected those expected in the discipline, or register, of science, as well as alternative language patterns used to realize the cyclical explanation genre in science. The language features used in high-scored samples were more aligned with those of the discipline compared with the low-scored samples. Although the low-scored samples revealed that students possessed some valid scientific understandings, these understandings were not as evident due to the students' limited use of language features commonly found in the science register. This work fills important gaps in the literature concerning the contribution of lexicogrammatical resources in conveying elementary students' science knowledge through written explanations.