A Really Cool Illusion

Someone named Sarah Snook sent BBC this photograph, which she entitled, "Footprints at beautiful Whitehaven Beach, Australia."
Do the feet seem to be indented into the sand or are they popping out of the sand toward you? Most folks will see them popping out. This is because the human eye, for some reason I can't remember, has a sort of default mechanism that "reads" the sunlight always coming from the left. In this picture the light is coming from the right, fooling the eye.

Now, click on the picture and see what happens when it is turned upside down so that the light really is coming from the left. Are the feet now indented or popping out at you?

Weird, huh? And cool, too!