Tools are usually classified according to the type of job they perform. Recognizing that each tool has a specific purpose is important. Nails and screws, for example, are used to join things together. Most hand tools are simple machines: devices that overcome resistance at one point by applying force at another point. Machines are designed to move or manipulate things using less effort, thereby making work easier. Thus, most tools can also be classified as one or more of six basic machine types: pulley, lever, screw, wedge, wheel and axle, and inclined plane.
All cutting tools perform a similar overall function -- they trim or shorten -- but they do so in different ways. The blades on a razor or lawn mower are examples of wedges. To shave a face or mow a lawn, a wedge's sharp edge is directed perpendicular to the beard hair or grass. Scissors and shears have wedges for cutting blades, but they operate differently from wedges that cut with a simple slicing action.
A lever is basically a rigid arm that pivots on a fixed point, or fulcrum. Scissors and shears, both examples of levers, actually have two arms that pivot on a single fulcrum. The cutting action is generated by the arms pivoting and passing each other. The location of the fulcrum in relation to the blades and handles influences how much force can be generated when cutting. By positioning the fulcrum so that a scissors' arms pivot near the middle, the blades open about the same amount as the handles. Since cutting paper doesn't require much force, this is fine. For more difficult jobs, such as cutting through tough fabric, shears are more useful. Because the fulcrum is located much nearer the blade, the tool's longer handles open wider and generate greater cutting force. But there's a tradeoff: longer handles must be moved further in order to do the job.