Nothing to fear but fear itself: introducing recursion in lower secondary schools

Recursion is a powerful conceptual tool, nevertheless it is often considered too abstract and technical to be effectively proposed in lower secondary school. Here we present our experience in introducing 8th graders to recursive strategies where the active observation of recursive algorithms execution allowed pupils to understand the main features of a recursive process, to convince themselves that it works, and to unveil the mystery of its effectiveness. In fact, by focusing on the self-similarity of the input data and the possibility to delegate blindly the solution of subproblems, pupils were able to recognize the heuristic potential of recursion. We started with an unplugged activity using LEGO bricks where a recursive algorithm was executed by pupils, and we continued with an activity supported by a software tool we developed ad-hoc. The undertaking was concluded by the abstract consolidation of the basic concepts and properties which had come out during the previous activities.