The students in this vignette enact several math practices as they engage in discussion.Questions that make the mathematics visible, such as “Do you think these patterns are truefor all of the columns?” encourage students to investigate regularity (SMP 8) and attendto the structure of the hundred chart (SMP 7). By asking students to reiterate and add to one another’s ideas, the teacher provides opportunities for students to articulate those ideaswith increasing precision (SMP 6) and presses students to carefully listen to one another’smathematical reasoning. Attending to the details of one another’s reasoning prepares students tocritique that reasoning and construct arguments, which are key elements of SMP 3 (CCSSI 2010). Moreover, this form of student participation has been shown to promote deeper understanding of key concepts and increase student learning gains (Franke et al. 2015).