A mechanical pencil looks very much like a ballpoint pen, but the fine writing tip is of lead or graphite. The first mechanical pencil was invented in Britain in the early 1820s, and patented by John Hawkins and Sampson Mordon in 1822. A mechanical pencil opens just like a normal pen, but instead of refilling it with a new ink cartridge, a length of specially manufactured pencil lead is fed into the writing barrel. Once the mechanical pencil is closed, lead can be pushed through the barrel in small increments -- as it is used -- by clicking the tip of the pen, depressing a ratchet button, or twisting the cone of the barrel, depending on the model. The advantage of a mechanical pencil is that the lead is so thin that it's always sharp, allowing precise and uniform strokes without the hassle of constant sharpening. This makes the mechanical pencil ideal for architects, draftsman, engineers, and anyone else that requires the convenience of a pen with the flexibility of an erasable pencil. One leading manufacturer of pens offers a liquid lead mechanical pencil. The graphite in this case, is in solution, behaving much like ink as it rolls from the ballpoint-like tip. However, once the fluid is absorbed into the paper and dries, only erasable graphite is left. This mechanical pencil purportedly writes with the fluid motion of a pen, while offering all the advantages of a pencil. It retails for less than $3 (US dollars), and is refillable. Aside from the convenience of never having to sharpen a mechanical pencil, they are also environmentally friendly, saving wood and eliminating the wood shavings of traditional pencils. An exception to this rule would be disposable mechanical pencils. Fortunately, there is little reason to buy a disposable model when refillable models are so cheap. Many leading manufacturers of pens make mechanical pencils, and several types of lead are available from soft to hard. Mechanical pencils can be purchased in sets with a matching mechanical pen, or individually. They range in quality from plastic barrels to enamel, gold or silver, and prices vary accordingly.