Figure-ground perception

Description- The law of figure-ground perception states in organizing stimuli we tend to automatically distinguish a figure from its background.
 http://psychology.about.com/od/findex/g/figure-ground-perception.htm

For example in this picture, depending on what we choose to use as the background, we either see a vase or two man's faces.

Below is a real life example because if we choose to use the sky as the background we see buildings in the foreground, but if we choose to use the buildings as the background we see letters in the sky in the foreground.

 http://en.wikipedia.org/wiki/Figure%E2%80%93ground_%28perception%29