A working model straight from one of the few laboratories using this relatively new technique, Force Chains is an experiment to see and measure stress in a granular material. Photoelastic disks inside the Force Chains container show the distribution of stress as visible streaks of light. Use a magnet to move and apply two-dimensional force to the disks, and watch the network of stress lines change in surprising and brilliant ways. Scientists use experiments like this one to better understand the effect of stress on granular systems, improving everything from shipping materials to seismology.