Isotropic materials are materials whose properties remain the same when tested in different directions. Isotropic materials differ from anisotropic materials, which display varying properties when tested in different directions. Common isotropic materials include glass, plastics, and metals. On the other hand, fiber-reinforced materials such as composites and natural materials such as wood tend to display anisotropic properties. These materials can demonstrate great strength when force is applied in the same direction as the fibers, and much less strength when force is applied in the opposite direction (with or against the grain).