Ever come across a 3-D work of art on the sidewalk?Does it ever take you by surprise? It's because those chalk works mess with one's depth perception. Take the work "The Dungeon" (picture at the bottom of this paragraph) for example; people are crawling from ropes and scaling walls to try and escape this horrid place. This is all done by drawing shadows and creating angles in the picture that would mess with your mind. The art is also "interacting" with the public. A man climbing to safety by rope is drawn on the sidewalk. A man has approached the rope and is "helping" the man climb to safety. Whether the man is the artist of this work or not, we do not know, but we know he is interacting with the art. This messes with the mind of the people who are watching. First, they see it as a 3-D image, which confuses people. We believe it's real and we can interact with such things. With the addition of the man and the rope, we become further confused and actually believe we can physically interact with the art (rather than just stepping on it).
The beauty of this trick is to view it from the right angle. If viewing a 3-D image from the wrong angle, the drawing looks elongated. If viewed through the right angle, though, people can see a whole new image.
Hopefully no one approaches a vicious shark from the right angle!