Peer instruction is an active learning method which was primarily invented by Eric Mazur for the needs of university level physics. The effectiveness of this method stands primarily on the group discussion that was raised by the conceptual question of the so-called ConcepTest. In this paper we will introduce peer instruction and then we will see how to implement this strategy in the teaching of elementary level mathematics using the Pythagorean Theorem. The following text will be also supplemented by examples of specific ConcepTests and it will be accompanied by statements of pupils about peer instruction.