One of the main differences between up and down in the world is the predominant direction of light. While not necessarily directly overhead, the sun’s rays generally come from above rather than below. This fact opens an important way in which we, and many other animals, can recognise solid three-dimensional objects. It works in reverse.
The photograph of moon craters is printed upside down. If your eye (well, to be more precise, your brain) works the same way as mine, you will see the craters as hills.
Turn the book upside down, so that the light appears to come from another direction, and the hills wil turn into craters that they truly are.
(Dawkins, 2004)