One intriguing question asked over the past several years is "how do people see?" Not the physiological process of vision but rather the neurological processing that takes place in the brain when viewing a photograph. Below is an image of an evening sunset over the Chiricahua mountains with a single illuminated cumulus cloud. It is displayed both right side up and upside down (and reversed, keeping the light from the same direction). In both images my eye is immediately drawn to the interface between the ground and the sky. In the upper image the ridge line of the Chiricahua mountains (including Cochise Head) is at the bottom of the photograph and black silhouette outline of the ridge top provides a nice demarcation between the ground and sky. While the lower image, the black silhouette of the ridge is at the top of the photograph. In this instance the tan of sky now becomes the "ridge line" and the "ground" with a black sky. But as the eye scans down the image, the cloud reorients the mind reminding the viewer it is sky not ground.