Most people think that teaching activities of primary school teachers can be performed very easily by layman. Primary school teachers teach students elementary knowledge which ordinary adults already have learned and understood, and therefore primary school teachers can be substituted by ordinary adults who want to teach children. Most people presume that elementary knowledge which can be understood easily can be taught very easily. But, contrary to this generally accepted idea, because primary school teachers are supposed to teach elementary knowledge, their teaching activities are very difficult. This research explored the nature of teaching activities with constant reference to primary school teachers, and found the predicaments that most teachers, including secondary school teachers and university professors, are confronting with. That predicaments can not be expected to be overcome by ordinary adults who have understood the subject matter knowledge, in case that they didn’t appropriated the principles of teaching. Especially, the teachers’ predicaments are used to emerge frequently when teacher must to treat elementary knowledge. In this point, contrary to common sense, primary school teachers are expected to have special abilities that can overcome the teachers’ predicaments, and therefore, to be best teachers.