Mechanical anchors are used to attach objects or structures to materials that cannot reliably support the required load with only standard fasteners or adhesives. Anchor are typically needed due to the limitations of the material's physical properties or structure. For example, concrete is not malleable enough to form a thread with a conventional screw; drywall material is too particulate to support even a moderate load on a single drywall screw; hollow walls will not allow access to both sides to mate a bolt with a load-distributing washer and nut combination.