A knife is a hand tool for cutting things into pieces, or
occasionally for scoring cuts in something without actually cutting through it.

(For convenience, in this Entry, the material to be cut will
be called the stuff).

Structure and Method of Use

A knife is usually made up of two parts, the blade and the
handle.

The handle is designed to allow the user to apply pressure
to the blade in comfort, and so is usually reasonably rounded and of a size
which is easily held in the hand. Some knives have their blade and handle made out
of the same material, e.g. flint or metal. Many use different materials, as the
differing desired characteristics of blade and handle can often be best met by
two different materials.

The blade is a structure designed to allow the application
of pressure to as narrow a strip of stuff as possible. (If a tool is designed
to apply pressure to the smallest point possible, then you have an awl, not a
knife). The narrowness of the blade gives a very high effective pressure (force
per unit area) with a relatively modest application of force. This pressure
causes stresses in the stuff and the bonds between its molecules to break,
splitting the stuff. This is called a cut.

The knife is often moved backwards and forwards to use local
stickiness (or deliberate roughness, often called serrations) at different
points on the blade edge to apply an additional force to the stuff,
perpendicular to the main applied cutting pressure. This additional pressure
further encourages the breakage of molecular bonds.

The effective use of a knife requires the application of a
equal and opposite force on the stuff. This is often provided by a hard surface
such as a chopping board. Sometimes the stuff itself provides the required
force, as would occur if a knife is used to carve initials on a dead tree.
Someone, somewhere realised that the force could also be applied by another
blade, and scissors were invented.

A knife is limited in what it can cut by the sharpness of
its blade, its hardness and the strength of the person using it. If the stuff
is harder than the blade, the blade will deform faster than the stuff will be
cut, rapidly thickening the blade and making little or even no permanent
impression on the stuff. This is called blunting the knife.

A knife can be sharpened by removing molecules from the edge
of its blade in such a way as to create two acutely intersecting planes giving
a very narrow edge. This is often done by holding the knife against a fine
abrasive surface at the appropriate angles, and moving the knife or the surface
(or both) until the required removal of molecules is completed. A lubricant may
be used to help this process and give a smooth edge to the blade. Water or
mineral oil are commonly used for this.

Another limit on what a knife can be used to cut is imposed by the strength of the blade, which
can break if the applied force is greater than it can stand. This can be
dangerous.

Some knives are designed to cut very soft materials. These
knives usually have very rounded edges, as there is no need for them to be
sharp. Examples include butter and fish knives.

The sharpest knives are used in eye surgery and their blades
are made of glass.

Some knives have blades with a variable relationship with
their handles. These include pen (or clasp) knives, which have a hinge in them
allowing the blade to be folded into a groove in the handle. An elaboration of
this is the flick-knife (or switchblade), which contains a spring that is
compressed when the knife is closed and that flings the knife open when a
button is pressed. Other knives, such as the Stanley knife, have disposable,
replaceable blades and these can often be retracted into the handle when the
knife is not in use.

There are many forms of knife, each suited to their task and
shaped by thousands of years of design based on the experiences of their users
and makers. New materials continue to extend the possibilities of knife design.
The story of the knife is not over yet. Oh no.