We study how elementary school pupils give sense to the moves of a mobile robot in a mathematical game. The game consists in choosing 3 numbers out of 6, whose sum is a given target number. The robot moves on a game board have been implemented to provide pupils with a tangible feedback about their answer. We have studied strategies of pupils to solve the problem and their evolution. Our methodology included interviews, aloud verbalization and video observations of 28 pupils in grade 1 and 2 while they are playing. The pursuit of a mastery goal encourages a trial and error strategy for only some of the pupils. We conclude that some aspects of the moves of the robot, like its position, are perceived as a form of help and not as a threat, even if they are only partially understood.