SELF- SHARPENING TEETH sea urchins don't just use their teeth to chew up food like kelp. The spiny marine animals also use their chompers to carve out nooks in rocks where they nestle to protect themselves from preda tors such as sea otters and some species of sea stars. Even though they chew through rock, urchins' teeth never get dull. Scientists have finally learned how that's possible. A has five teeth made of calcite that sea urchin meet at the opening of its mouth. Calcite is a mineral composed of the elements calcium, carbon, and gen. Pupa Gilbert, a physicist at the University of Wisconsin, recently used a scanning electron microscope to get a super-close look at the teeth. She noticed the calcite crystals inside them that were irregularly shaped and separated by thin membranes. When a tooth layer cracks from too much wear, it off along a membrane. simply snaps They're sort of like perforated paper, with membranes where they should break," says Gilbert. Each break exposes a fresh, pointy calcite surface, so the tooth stays sharp, and the urchin can keep Valerie Ross grinding away.