Pupils from the age of 12 to 16 can occupy themselves for hours with their skate-board, walkman or the video-recorder, while their concentration during a lesson of mathematics vanishes often within a few minutes. What could be the cause? Isn't it possible to keep their attention during the lessons? Of course, I'm not the only one posing this question. Many solutions have been proposed in the past for this concentration and motivation problem. In this article I just want to add another one. I do not have the illusion that I am presenting the ultimate solution, simply because it does not exist. For a good teacher having one solution only is not enough. He or she has to combine and diversify the various ideas continuously. The keyword for the lessons of a good teacher is variation.