Perhaps you have noticed something like the following in very young children. Matthew is playing in the sand box, pouring the dry sand from his shovel into the bed of sand. The playground attendant has asked the children to keep the sand inside the sandbox frame. But Matthew becomes interested in the contrast between the blond sand and the green grass so he tilts his shovel to let the sand trickle to the grass. The alert attendant speaks firmly to Matthew, “Stop, in the box please.”