When the focus of instruction is on learning to use mathematical tools--such as a ruler or a calculator--as a procedure, students tend not to incorporate their own mathematical reasoning. This unreflective use of tools hinders students' questioning of the reasonableness of their answers. This article describes how children expressed their mathematical agency by challenging the knowledge authority of a mathematical tool: the protractor. The first author--a researcher in the classroom--and the second author--the classroom teacher--encouraged students' critique of the traditional protractor. Students expressed mathematical agency by (1) challenging the protractor's effectiveness in helping them measure angles and (2) by imagining an alternative tool. The students did not build their imagined tool. Instead they used their imagined alternative tool to help them better understand how to use a conventional protractor. The authors first define student mathematical agency as an important component of student learning experience. Then they describe how students' initial use of a traditional tool presented an opportunity to foster the expression of their mathematical agency. Then they illustrate how in turn, the expression of their mathematical agency helped students critically consider the traditional tool to make sense of it.