As a Mechanical Engineer, I have found the aesthetics and physics of rock stacking to be interesting. For people that do not know of this art, rock stacking is when you take rocks and stack them on top of each other to produce a structure. Rocks tend to(but not always) have minimal flat edges, are stacked vertically and only 1 rock tends to be used as a base. This art requires the creator to find the center of mass of each rock and place it perfectly on top of the previous rock in order to keep it stable. The end result is a small tower that looks as if it is defying the laws of physics and should fall over any second.
In Boulder there is a local resident(Michael Grab) that goes along the Boulder creek and stacks rocks. His work is very impressive, and I would recommend for people to keep an eye out for it during the summer. What makes his work even more impressive for me is that he is also dealing with wet slippery rocks that are forced to account for water spray from the creek. Here is a link to his work: http://www.gravityglue.com/ And here is a link to an article about him: http://twistedsifter.com/2013/01/rock-balancing-art-by-michael-grab/
I find this art to be impressive since I am able to understand how difficult it must have been to create it. This art makes you pause and wonder how the rocks are able to keep their balance. Even if you draw a FBD in your head it might still seem impossible.