Communication in school is a mutual interchange of information among teachers and students, studens and students during the educational process, i.e. students have an active role in the whole process (Mareš & Křivohlavý, 1995). Using explanation in a mathematics classroom is a normal procedure, but its roles and forms vary. Predominantly explanation is seen as a tool for describing relevant phenomena, developing students' logical thinking, and guiding students by inductive judgement to generalising. It leads to clarifying interrelations, demonstrating and justifying (Skalková, 1999, p. 172).