Informal formative assessments (IFAs) are classroom interactions teachers use to gather information about their students' learning, interpret it, and act on this information to achieve academic goals. One of the responses science teachers can enact as the result of IFA is constructing explanations to clarify science concepts to their students. Yet, there is scant research on the characteristics of IFA and the explanation practices that could support English Learners in their language and science knowledge development. In this study, we explored the IFA practices of one elementary and one secondary science teacher as they engaged with their students in constructing science explanations. Key findings indicate that teachers frequently co-constructed explanations with their English Learners as a response to IFA interactions. Yet, both teachers used explanations as a response to IFA unsystematically and not as a science development goal. In fact, both teachers used IFA mostly to assess students' language knowledge rather than their science understanding. Even when explanations were constructed to support English Learners' language, these explanations were only constructed orally and teachers' corrections mostly focused on language form (e.g., vocabulary, correct syntax) rather than on language function (i.e., how language is used to construct clear explanations). Finally, our study shows that for explanations to be effectively used with English Learners, teachers need to conduct adequate sampling of their students' science knowledge and language skills by creating structures that allow them to participate in classroom interactions. Implications for IFA and science explanations research and practice are discussed here.