How do we teach children to express and communicate ideas in a formal and informal mode? What type of language do they need in a concrete context? How should they determine a proper level of formalization of their descriptions? In an attempt to explore these issues we carried out a pilot experiment in the frames of the DALEST European project whose goal was to create environment for stimulating the 3D geometry understanding of young students and to assist them in developing some fundamental mathematical skills including spatial visualization and articulating ideas. The pilot experiment was carried out with 5th graders from five Bulgarian schools by means of specially designed educational scenarios and the Cubix Editor (a Logo based application for manipulating unit-sized cubes). The children were given tasks to describe compositions of unit-sized cubes and to build such compositions by means of the Cubix Editor when given their descriptions by peers. The students experienced the whole...