This paper indicates that prospective teachers' familiarity with theoretical models of students' ways of thinking may contribute to their mathematical subject matter knowledge. This study introduces the intuitive rules theory to address the intuitive, "same sides-same angles" solutions that prospective teachers of secondary school mathematics come up with, and the proficiency they acquired during the course "Psychological aspects of mathematics education". The paper illustrates how drawing participants' attention to their own erroneous applications of "same sides-same angles" ideas to hexagons, challenged and developed their mathematical knowledge.