An environmental surface , I have suggested, is the interface between a substance and the medium in which an observer gets about. The variables and the features combine to yield an enormous variety of surface shapes. The texture and the color that characterize a surface are persistent and do not change much in the course of environmental events but the shape of a surface is persistent only to the extent that the substance is solid and thus resists deformation, or that it is undisturbed. The surface is deformed when the substance is viscous or visco-elastic, and is changed rapidly and completely when the substance is liquid, or granular. The non-rigidity of a substance is made evident by the change of shape that occurs when it is acted upon.
Children are curious about and observant of such shape changes.
It is now possible to suggest what the difference is between a substance and an object. Both have a surface with a visible texture and color but only an object has a persisting shape. On these constants are superposed minor changes of shape that constitute her behavior and her expressive movements. An object, then, is a persistently and characteristically shaped surface. A blade of grass, a tree, and even a house are objects although they are not detached from the ground.