In mechanical mouse the ball is use to control the arrow of the mouse on the screen. Inside near are two balls they send electric signal through the wire! In optical mouse which are most adjectives these days are used everywhere. They have small red light prearranged as LED (Light emitting diode) emit light on the underneath and that send signals to the computer. It is beeter than mechanical one because -it has few likelihood of signal failure -dust can't get into it -smoother than the mechanical -especially the mouse wad is not required!