Unlike the mechanical-trackball "mice of yore" that skip, stick, and need to be cleaned, the Microsoft D68-00007 TrackBall Explorer introduces an entirely new trackball featuring optical sensor technology. Reading through dirt, dust, and fingerprints, the optical sensor tracks the movement of the ball 1,500 times per second, giving you smooth, precise control. The Trackball Explorer comes with two additional buttons located on the side of the device--for a total of five buttons--for moving forward and back quickly in your browser, or you can program them. It also features a wheel for efficient scrolling and zooming and is ergonomically designed to support the natural position of your hand as you use your index finger to control the trackball.