It is a fun surprise for people. The easiest one you can do is a pointed rock on top of the duck, and then a "signpost" rock at the shadow tip. If you're hiking trails in northern California and see a stack of rocks, and off to the side where the shadow is you see another rock that is distinctly different (in shade or composition) sitting in the arc that the shadow traces, that could very well be one of my "date ducks."