A knife (plural knives) is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knife-like tools were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools.[1][2] Originally made of rock, bone, flint, and obsidian, knives have evolved in construction as technology has, with blades being made from bronze, copper, iron, steel, ceramics, and titanium. Many cultures have their unique version of the knife. Due to its role as humankind's first tool, certain cultures have attached spiritual and religious significance to the knife.[3] Most modern-day knives follow either a fixed-blade or a folding construction style, with blade patterns and styles as varied as their makers and countries of origin. A blade is that portion of a tool, weapon, or machine with an edge that is designed to cut and/or puncture, stab, slash, chop, slice, thrust, or scrape surfaces or materials. A blade may be made from a flaking stone, such as flint, metal (usually steel), ceramic, or other material. Stone blades were among the first human inventions, dating to the Oldowan culture in human prehistory - see stone tool - and fundamentally changed human existence by aiding the transition from a prey species on Earth to a predator species, as well as giving humanity a cutting tool that could be used to shape other tools. Since then, blades have been used for utility purposes (food preparation, craftwork, outdoors sport, etc.) and for combat for millions of years. When used for food preparation, the main uses are slicing (cutting by drawing the blade across the object, applying light pressure) and chopping (cutting by pushing the blade through object), with some piercing (using the tip to cut through the surface).[1] In combat, a blade may

be used to slash (i.e. to cut with the edge, generally in a swinging motion), puncture or stab (the blade is plunged into the opponent, starting with the tip and proceeding as the blade enters more deeply), thrown (i.e. by throwing the knife so the point enters the target) or propelled (i.e. a ballistic knife).[2][3] Blades may be used to scrape, moving the blade sideways across a surface, as in an ink eraser, rather than along or through a surface. The ability of a blade to cut arises from the concentration of the force applied to the blade onto a very small area, resulting in a high pressure on the matter to be penetrated. A serrated blade (a blade which has many small "teeth") takes this further as each individual tooth concentrates the force on a smaller area which helps cut through denser materials. A jagged knife can cut through objects solely with a sliding motion with little pushing force, this is useful, for example, in bread knives. Some bladed weapons or tools have curved blades. As a rule the blade must be made of a substance which is harder than (or as hard as) the material it is intended to cut. If this is not the case the blade will either be unable to cut (as it absorbs all the energy and is damaged) or will wear away very quickly (if it is hard enough to transfer enough of the energy to damage the material). The material must also be tough enough to last (e.g. glass is very hard but it shatters easily and thus is not very effective as a material for a blade). Heat treatments, which increase hardness for better edge-holding, reduce the material's toughness. A balance must be found between the sharpness and how well it can last. Methods that can circumvent this include differential hardening. This gives an edge that can hold its sharpness and a body that is tough.