A shape is the form of an object or its external boundary, outline, or external surface, as opposed to other properties such as color, texture or material composition. Psychologists have theorized that humans mentally break down images into simple geometric shapes called geons.Examples of geons include cones and spheres. Some simple shapes can be put into broad categories. For instance, polygons are classified according to their number of edges as triangles, quadrilaterals, pentagons, etc. Each of these is divided into smaller categories; triangles can be equilateral, isosceles, obtuse, acute, scalene, etc. while quadrilaterals can be rectangles, rhombi, trapezoids, squares, etc.