This article aims to describe a type of formative assessment, MAP Classroom Challenge, which has been introduced in mathematics classrooms in recent years. MAP, or the Mathematics Assessment Project Classroom Challenges (formerly known as Formative Assessment Lessons), are developed by teams of math educators from the Shell Centre for Mathematical Education at the University of Nottingham and the University of California, Berkeley, with the support from the Bill & Melinda Gates Foundation. The overarching goal of Classroom Challenges is to support teachers in administering effective formative assessment by identifying student misconceptions of main mathematical ideas and helping students develop better understanding. The design of Classroom Challenges stems from the Common Core State Standards in Mathematics and it is based on the seamless fusion of the standards for mathematical content and the standards for mathematical practices. Furthermore, this article demonstrates the positive impact one Classroom Challenge had on revealing and subsequently eliminating student misconceptions related to certain fundamental algebraic concepts.