During the past few years, several of the authors have incorporated student problem posing as a regular instructional feature in their classrooms. When they offer their students the opportunity to construct their own problems, particularly during the course of an entire school year, they create many novel tasks. Student-created tasks not only serve as teaching tools for our students but also are excellent formative assessments for us, providing insight into students' mathematical conceptions and interests. In this article, the authors describe classroom interactions surrounding the creation and implementation of the Reverse and Add to 100 task, developed collaboratively with third graders to strengthen their understanding of place value and multi-digit addition. Their explorations of Reverse and Add to 100 promoted student reasoning and problem solving, fostered meaningful mathematical discourse, elicited and made use of student thinking during instruction, and supported productive struggle.