We contend that the classroom-discourse routine of IRE/F (teacher initiation, student response, teacher evaluation/follow-up) is a genre of argumentation that is both collaborative and implicit because teachers and students cooperate during IRE/F exchanges not only to make and mirror knowledge claims but also to suppress justification for those claims. Analyzing consecutive IRE/F exchanges drawn from a first-grade geometry lesson, we propose that the implicit argumentation of IRE/F may conflict with students' learning of justification-centered forms of reasoning that are promoted in the cultural communities of academic disciplines. To highlight the argumentation and identification practices at work in IRE/F exchanges, and to expand teachers' options for responding to students, we offer the flexible heuristic WAIT (Welcome, Analyze, Include, Thank), which transforms the third IRE/F turn into an opportunity for explicitly analyzing students' argumentation practices and including students in disciplinary, school, and classroom communities.