Segmented hinges in the long, thin bones of fish fins are critical to the incredible mechanical properties of fins, and this design could inspire improved underwater propulsion systems, new robotic materials and even new aircraft designs. Fish fins are not simple membranes that fish flap right and left for propulsion. They probably represent one of the most elegant ways to interact with water. Fins are flexible enough to morph into a wide variety of shapes, yet they are stiff enough to push water without collapsing. The secret is in the structure: Most fish have rays – long, bony spikes that stiffen the thin membranes of collagen that make up their fins.