A common goal for mathematics teachers at any grade level is for "all" students--not just future mathematics teachers--to develop deep understanding of the mathematics they are learning and using. To that end, this lesson can be used in an upper-level high school mathematics course to introduce the concept of a radian in a way that encourages student reasoning and reflection. This discovery-based lesson is designed to provide students with opportunities for active engagement and productive struggle as they construct angles and formulate a rich conception of the radian.