The main question we try to answer in this paper is whetherstage-like progression in cognitive development can beexplained by transfer of cognitive skill among tasks. Wefocus on the following question: To what extent does trainingon one task improve the performance on another task? Thetasks are Piaget’s (1959) Balance Scale Task and NumberConservation Task, and a task that we will call the Une-Sentence Task, which is taken from Karmiloff-Smith's (1979)experiment on the acquisition of determiners in French. Were-implemented already existing models within theframework of the PRIMs cognitive architecture (Taatgen,2013). Each task was subdivided in certain stages related tothe complexity of the problem-solving strategies. We showthat mastery of a certain stage of a problem becomes easier ifa higher stage of another task is mastered first.