In this article, three instructional situations demonstrate the value of using an "unknown" student's work to allow the advancement of students' mathematical thinking as well as their engagement in the mathematical practice of critiquing the reasoning of others: (1) introducing alternative solution strategies; (2) critiquing inaccuracies in others' work; and (3) introducing new mathematical concepts. The article includes three recommendations for using an unknown student's work: (1) Keep your math goals in mind; (2) Do not limit alternative strategies to "better" strategies; and (3) Use accessible student work.