This paper presents a cognitive model for the routine evolution of skills in a photocopying task. In the classic study by Agre and Shrager (1990), it was found that although speedup in performance conformed to the power law of practice, microgenetic activities of the subject showed multiple stages of qualitative changes as the subject repeated the same task. The evolution of skills was shown to be more appropriately characterized by a "situated", dialectic relationship between the subject and the environment than by a set of general mental mechanisms as in traditional theories of skill acquisition. This paper attempts to bridge the gap between these two perspectives of skill acquisition by showing how the dialectic relationship can be characterized by a traditional information processing model by making a few additional assumptions of the relationship between the human and the environment. A model based on the ACT-R architecture was constructed, which provided good fits, both qualitatively and quantitatively, to the data collected by Agre and Shrager. The model demonstrated how multiple mechanisms could be implemented in a cognitive architecture to explain both qualitative and quantitative changes at both the macro and micro levels of analysis. Implications to human factors research are discussed.