Classroom talk is frequently limited, often used to check student comprehension rather than develop thinking. In classrooms with higher numbers of students living in poverty, teachers talk more and students talk less (Lingard, Hayes, and Mills 2003), positioning students as passive learners, which is problematic to developing critical thinking. Over two months, the authors' research team observed the materials engineering unit, "A sticky situation: Designing walls" (Engineering is Elementary [EiE] 2008), in which students capitalized on multiple opportunities to develop critical thinking through talk. These 15 students (nine African American, six Latinx; 12 female, three male) and their teacher, from an urban, economically disadvantaged school, engage in the unit as they read a storybook featuring a character in China who protects her garden from rabbits, investigate the properties and purposes of earth materials, and use some combination of sand, soil, clay, and water to create a "good" mortar. As students engage in this unit, they are regularly afforded opportunities to expand their thinking through student-engaged talk. Herein, the authors highlight three epistemological messages effective in promoting student-engaged talk and critical thinking throughout this engineering unit. Next, they provide examples of the teacher delivering these messages and students applying the meaning in discussion.