We present results of a research developed with first semester students from a Colombian Public University based on classroom intervention in a precalculus laboratory course mediated by an interactive mathematical software. We characterize and exemplify the cognitive skills of explanation, justification, argumentation and validation, using a methodological tool of analysis that integrates the ckç model of Balachef and Margolinas to Toulmin's model. This tool allows us to identify some elements that characterize each of these skills and allow us to offer a structure of analysis that we present as a proposal for the design of activities in research, based on classroom interventions that promote the development of demonstration skills.