A knife is an example of a simple machine called a wedge. A wedge is made up of two inclined planes placed back to back to form a sharp edge. When the edge is placed between two objects and force is applied, the wedge drives the objects apart.
The wedge is one of the six classical simple machines defined by scientists during the Renaissance. The others are the lever, the wheel and axle, the pulley, the inclined plane and the screw. The idea of the simple machine can be traced back to the work of the Greek philosopher Archimedes, who discovered that applied force could be amplified by using the mechanical advantage of a lever.