Rock Balancing Optical Illusion

These fine rock creations are the art work of Bill Dan of San Francisco, California. The title above suggests that these rocks are balanced, just sitting there waiting for someone to topple them over. What do you think? When you are done pondering how this was accomplished you can click the link below the picture.

Bill Dan
Copyright 2005
used w/permission

How about the photo below. It would seem impossible to believe that these are balanced. Look at the center stack, there is no way those rocks are balanced. Right?

Bill Dan
Crissy Field
Copyright 2004
used w/permission

So what do you think is the mysterious force holding these rocks in place?

Update: Click the image to go to Bill's site and read his explanation for yourself.