This study concerns the question of how teachers can help pupils to perceive the concept of weight (gravitation). Fifth and ninth graders were asked in a paper-and-pencil test to compare the weight of two objects suspended in a pulley-in-balance half a year after the learning intervention consisting of three successive pulley-in-balance demonstrations. The understanding, that some of the fifth graders seemed to have formed from the weight concept after the teacher-independent learning intervention, had almost totally vanished in the delayed study. In the demonstrations the pulley served as an efficient prosthetic device for thinking, helping pupils to pay attention to the behaviour of the whole system instead of looking only at the separate objects. In the ninth grade about 45% of the pupils achieved general understanding about the pulley in balance by transferring the scientific explanation from the pulley demonstrations to the paper-and-pen test. A learning-with-conflict model is proposed based on pupil's alternative explanations about a discrepant event. These explanations are then challenged with a conflicting event. When pupils see and understand how the concept works in different contexts it is possible for the pupils to reach a context independent conceptual change.