In the first part of this article, we briefly describe a study which had the objective of finding out the benefits of introducing in the classrooms of elementary education, a computer, a projector and computational software designed with well founded didactical principles. However, our main purpose here is to show the effect that the type of the teacher' s behavior has on the cognitive development of his students within this teaching situation. The results revealed the strong influence of the teacher' s instruction and interaction modes, and point to this as a critical factor in the teaching and learning process of any pedagogical model. However, teacher' s behavior is, in large measure, a reflection of his content and pedagogical knowledge, but especially of a knowledge that intertwines both, called Mathematical Knowledge for Teaching. Thus, we have to focus constantly on this more essential aspect, if we hope for significant advances in education.