Screws are fasteners made typically of metal, with a set of threads and a head on the top containing a shape that allows it to be driven with a tool (usually a screwdriver, but sometimes hex heads are also used). Unlike bolts, screws generally do not require a nut on the other end, as they are driven into the material. Body screws typically have a replaceable receptacle, commonly referred to as a body nut, to fasten trim and associated bodywork to the chassis. Body nuts do not require a wrench to secure. Screws keep many parts attached to the car, including body panels, undercarriage protection, and engine components. Screws subjected to the outside elements can wear and break over time, due to oxidation or fatigue.