The population of emergent bilingual students (EBs, or English Language Learners, ELLs) is growing steadily in the United States. One common approach to language instruction for EBs is to expose students to a lesson's content vocabulary before teaching the lesson to build students' word banks and background knowledge. An example of this is the Sheltered Instruction Observation Protocol (SIOP). In this article the authors explore vocabulary instruction in the context of the "Next Generation Science Standards" (NGSS), which center students "doing" science and "figuring out" explanations for scientific phenomena (i.e., engaging in scientific sensemaking) rather than simply learning about science. This instructional shift challenges all teachers, but it may especially challenge teachers of EBs who are trained in strategies like SIOP. With an emphasis on "figuring out," how and when can vocabulary best support students' science learning? Should EBs vocabulary be taught "before" the science lesson, as suggested by SIOP and other language acquisition models? Or should teachers encourage students to work with their own terms and ways of expressing ideas--and for how long?