“Suppose I pick up my box to help you see what is really going on. Now does the box appear to be moving nearer and further? Perhaps it seems like the box is shrinking and expanding?

“I am showing you this illusion to demonstrate the ambiguity of visual information. When an object becomes smaller, it could really be getting smaller rather than farther away. But magical boxes like the one I am holding here are rare, and it is far more likely that a change in size really is caused by a change in depth.

“Now that we understand size constancy, we can return to our original figure and explain the ambiguity there.”