Constructive Triangles

Constructive Triangles let the child discover that every figure with straight sides can be built out of triangles. The child slides one piece against another until the edges meet with no gap, looks at the figure that has appeared and then takes it apart to try a different pairing. Sliding the pieces together edge to edge turns the same few triangles into a rhombus, a trapezium and a larger triangle, and the hand grasps what geometry will later put into words. A gap or an overhang says at once that the wrong sides were joined, so the correction comes from the material rather than from an adult. Sensorial preparation for plane geometry, for children from three years.