In this work, we discuss the children’s comprehension of the concept of geometric shapes with an emphasis on the shape of a triangle. To be working with children concretely it is necessary to know what constitutes their development, more precisely, to know how children get to know the world around them, which is by play. Through play, children learn mathematical concepts and skills. Adopting basic mathematical concepts affects the development of all psychological functions, thought processes, properties of thinking and forms of thinking. The pedagogies of Maria Montessori, Jean Piaget, Jerome Bruner and Lev Vygotsky are also referred to. For learning geometry, P. van Hiele’s model of geometric thinking and E. Rosch’s prototype theory are described, as well as the research that proves these theories. According to the model of geometric thinking, preschool children are at the level of visualization, and according to the prototype theory, children can most easily recognize a triangle that is a prototype, that is, an equilateral triangle with one base horizontal and the opposite vertex above the base, not below. In the continuation of the work, there are activities listed to encourage a faster transition from one phase of understanding geometric shapes with an emphasis on triangles to another phase. The activities are divided into games, problem-solving and specific mathematical problem-solving exercises, worksheets, and the book Leo and Triangles is cited, which serves to introduce the STEAM area.