A mechanical pencil often looks more like a ballpoint pen than a pencil. You open it like a ballpoint pen by unscrewing or separating the top and bottom, but instead of changing an ink cartridge, you insert a new thin graphite lead. The pencil contains an internal mechanism which propels this lead through the tip, usually by the user clicking the top or twisting the pencil cylinder. These pencils commonly include a rubber eraser on the end, although it may be hidden under a cap. Most mechanical pencils can be refilled, but some are disposable.
Mechanical pencils create lines of consistent thickness and are often favored for technical drawing and graphic art. Another advantage is that they do not need sharpening, except for those with thicker leads, which are typically only used by graphic artists. Many people prefer mechanical pencils for these reasons, and also because the pencil can be used year after year and does not have to be replaced.
Several types of mechanisms are available in mechanical pencils, with the most common being the ratchet type. In these pencils, lead is pushed forward in small increments when you click a button on the pencil end or side. This mechanism includes small jaw pieces which separate and allow the lead to move forward, and a rubber device at the tip which holds the lead in place to stop it from falling out or sliding back upward.
Another type is a screw-based pencil, where you propel the lead forward by turning a screw, which pushes a slider down the pencil barrel. Twist pencils allow you to move the lead forward and backward for a precise length of lead, and to retract it when finished to keep the lead tip from breaking.
Like other modern pencils, the leads are made with graphite instead of lead. Graphite is a naturally-occurring form of carbon that is not toxic. The most common lead sizes for mechanical pencils are 0.5 and 0.7 millimeters. Lead can be purchased in packages of about 10 to 30, which are available at department stores and office supply stores.