How does cartilage do so much for so long, and why does it eventually give way? In this lecture, I will show that the answer lies in a beautiful piece of physics: cartilage is a network of microscopic fibers living right on the edge of a phase transition between soft and stiff. Small changes — a little less collagen, a little less swelling pressure — can produce enormous changes in how the tissue responds to load. The same principle that protects your joints for eighty years can, once crossed, lead to sudden failure. And the same principle, we are beginning to learn, can guide us in designing a new generation of resilient, lifelike materials.