Learning to play tennis is difficult. It takes practice, but it also helps to have a coach--someone who gives tips and pointers but allows the freedom to play the game on one's own. Learning to act like a mathematician is a similar process. Students report that the process of proving the inscribed angle theorem is challenging and, at times, frustrating. Most are not used to the open-endedness of the proof process, which contrasts with more traditional approaches in mathematics that commonly place more emphasis on algorithms, rules, and procedures. Having prospective teachers find the inscribed angle theorem for themselves can foster mathematical reasoning. (Contains 9 figures.)