The fan blades in modern aircraft engines are coated with a protective ceramic to keep them from overheating. When the metal heats up, it expands, and the ceramic coating expands with it. But when grains of sand are inevitably sucked in and contact the many thousand-degree blades, they melt and make glass. The glass not only breaks down the coating when hot, but when it cools, it forms an inelastic layer on top of the protective coating. When the blades heat up again, the glass doesn't expand and breaks off the ceramic, shortening the life of the engine.