The Three Mountain Problem
On Piaget's view, thought in the preoperation period is egocentric: children tend to be captured by their immediate concrete perceptions and find it difficult to adopt alternative viewpoints.  The three mountain problem illustrates this tendency.  Children are asked to draw how the mountains would look from the doll's point of view.  Typically, 3- and 4-year-olds simply draw how the mountains look from their own viewpoint.  The problem is not that young children don't know that the mountains ought to look different from the other side.  In experiments where the mountains were surreptitiously shifted while children were led around the display, children were surprised to see that the mountains looked just the same from the opposite side.