This is a modern replica of the technique Gaudí used to design his buildings -- hanging little bags of birdshot from strings. Gravity would pull these bags downwards, giving even weight distribution and stretching the strings to form a model structure, thus showing him the shapes and angles his pillars would need to be. By photographing the model, then inverting the photograph, Gaudí could then see the model as it should look. This image is of the hanging model; below is the reflection in the mirror, showing how the building would look. A more graphic view of this technique can be seen here http://www.pbase.com/helenpb/image/151191168