In the rockiest, most inhospitable part of the ocean resides the mussel, a shelled mollusk for whom the relentless onslaught of wind, sand, and crashing waves is commonplace. But thanks to some 50 to 100 silky threads—known collectively as the byssus (and sometimes the "beard")—that grow out of their large, muscular foot, mussels latch onto rocks and corals with ease. The trick, researchers report tomorrow in Science, has to do with a special type of protein in the foot, known as a dopa protein, which can bind with varying degrees of strength to metals in the byssus. In the inner layer, the dopa proteins and metals remain loosely bound, enabling the mussel to sway with the ocean current like a rubber anchor that expands and slowly retracts. But the byssus' outer layer is studded with more tightly bound dopa-metal combinations that appear as knobby protrusions under a microscope (inset). These knobs maintain the anchor's flexibility but also make it very hard, a rare combination in nature. Engineers have long sought to create a similar stretchy/hard compound (think of cars that could crash and recoil like bouncy balls), but so far mussels have outwitted them.