Several authors manteined that when subjects solve problemes by analogy, they map the source onto the target by means of an incremental process which gradually leads, through the addition of new similarities, to the complete source-target overlapping. In Experiment 1, 2, and 3 subjects received the source and then were presented the target problem step by step. At each one of the six steps subjects had to evaluate the correspondences between the target and the sources. In the first experiment three kinds of source were used: neutral, superficial and structural. To avoid the evaluation to be conditioned by an interaction among the sources, two experimental conditions were added in the second experiment: the structural and the superficial source were presented between two neutral sources; in Experiment 3 the structural source and the superficial source were isolated. In Experiment 4 subjects first read the target and then the source - the structural or the superficial one - was presented step by step. Analogical mapping monitored through this similarity rating task showed a critical point where subjects suddenly recognized the isomorphism between the source and the target.