What goes on inside of a granular material like sand when an object moves through it? Individual grains will shift and may impact one another or simply slide past. Researchers use special photoelastic materials to see these forces in action. A photoelastic material responds to changes in stress by polarizing light, revealing areas of stress concentration. For an entire network of photoelastic beads, forces between the grains appear like a web of lightning. Individual strands are known as force chains. Bright lines indicate areas where grains are jammed against one another in opposition to the object’s movement. As the intruder is pulled against the force chain network, grains shift and new force chains form. (Image credit: Y. Zhang and R. Behringer, source)