The student quickly realizes that the strategy “the father is always 2 cm higher than his daughter” is not correct. This can also be achieved by exploring the other extreme where the original picture is blown up to poster size and the daughter is 100 cm high. How high will the father be in this poster? A student answering 102 cm realizes that the father and daughter are almost the same height which cannot be right. Once cognitive dissonance is present, the teacher can introduce the correct relation, constant ratio.