A not entirely unusual position among teachers is that they believe that they must first establish a peaceful classroom before they can begin to teach the subject. This research, shows how a proficient mathematics teacher teaches his subject and thereby creates a quiet and focused classroom and exerts effective leadership, just by teaching mathematics. The researchers observed a male mathematics teacher for almost half a year, i.e. one semester. The results of research present several patterns that the researchers saw during the observations of his teaching. The teacher showed an interest in each student's mathematical thinking and expressed explicitly how students were expected to learn mathematics. He also directed students' attention to mathematics and established a culture where all solutions were important in the teaching process. In the teaching process, he used multiple representations to motivate students and a lot of supportive expressions that made them feel that they were able to learn mathematics. He worked patiently to establish structures, and there was almost no disruptive behaviour. Students simply did not have time to interfere because they were so engaged in learning mathematics.