Finally, it should be noted that a great deal of manipulation occurs for the sake of perceiving hidden surfaces. I can think of three kinds of such manipulation: opening up, uncovering, and taking apart. Each of these has an opposite, as one would expect from the law of reversible occlusion: closing, covering, and putting together.
Opening and closing apply to the lids and covers of hollow objects and also to drawers, compartments, cabinets, and other enclosures. Children are fascinated by the act of opening so as to reveal the interior and closing so as to conceal it. They then come to perceive the continuity between the inner and the outer surfaces. The closed box and the covered pot are then seen to have an inside as well as an outside.
Covering and uncovering apply to a cloth, or a child's blanket, or to revealing and concealing by an opaque substance, as in a sandbox. The movement of the hand that conceals the object is not always so clearly the reverse of the movement that reveals it as it is in the case of closing-opening, however. The perceiving of hidden surfaces may well be more difficult in this case.
Taking apart and putting together apply to an object composed of smaller objects, that is, a composite that can be disassembled and assembled. There are toys of this sort. Blocks that can be fitted together make such a composite object. Taking apart is usually a simpler act of manipulation than putting together. Children need to see what is inside these compound objects, and it is only to be expected that they should take them apart, or break them apart if need be. After such visual-manual cooperation, they can perceive the interior surfaces of the object together with the cracks, joins, and apertures that separate them. This is the way children come to apprehend a mechanism such as a clock or an internal combustion engine.
Active locomotor behavior, as contrasted with passive transportation, is under the continuous control of the observer. The dominant level of such control is visual. But this could not occur without what I have called visual kinesthesis, the awareness of movement or stasis, of starting or stopping, of approaching or retreating, of going in one direction or another, and of the imminence of an encounter. Such awarenesses are necessary for control.
Also necessary is an awareness of the affordance of the encounter that will terminate the locomotor act and of the affordances of the openings and obstacles, the brinks and barriers, and the corners on the way (actually the occluding edges).