A screw is a type of fastener, in some ways similar to a bolt (see Differentiation between bolt and screw below), typically made of metal, and characterized by a helical ridge, known as a male thread (external thread). Screws are used to hold things together as they pull or push the object. Screws may also be used for lifting heavy objects and to tighten things, like the vice in the image to the right. Because of the screw's ridges, a screwdriver is used to drive a screw into wood as it is turned in a circular motion.