In this report we investigate how pupils tackle an interactive problem-solving task recently proposed in the Bebras Challenge. The task comes across as fun and simple and we proposed it to all pupils from fourth grade up. We analyzed the data collected, produced by 18,486 participating teams while interacting with the local Bebras platform. The quantitative data were supplemented by interviews conducted after the challenge, in which students were asked to solve the question while thinking aloud. Regardless of their age, all the participants found a good challenge in the task and they spent a lot of time interacting with it. Almost all teams attempted a naive approach to the solution, misled by the superficial characteristics of the problem, and many insisted on attempting the naive approach without ever abandoning it; this behaviour clearly decreases with increasing age.