MOUSE

1] As you move a mechanical mouse by dragging it across a flat surface, a ball- made of rubber or ruber     over steel - protruding from the underside of the mouse turns in the direction of the movement. 2] As the ball rotates, it touches and turns two rollers mounted at a 90-degree angle to each other. One roller responds to back and forth movements of the mouse, which correspond to vertical movements onscreen. The other roler senses side ways movements, which correspond to side-to-side movements onscreen.

3] Each roller is attached to a wheel known as an encoder, much as a car's drive train is attached by its axles to the wheels. As the rollers turn, they rotate the encoders.
4] On the rims of each encoder are tiny metal contact points. Two pairs of contact bars extend from the housing of the mouse and touch the points on each of the encoders as they pass by. Each time a contact bar touches a point, an electrical signal results. The number of signals indicated how many points contact bars have touched- the more frequent the signals, the faster you're moving the mouse.The  direction in which the rollers vare turing, combined with the radio between the number of signals from  the vertical and horizantal rollers, indicate the direction that the mouse is moving.
5] Signals are sent to the PC over the mouse's tail-like cable. Windows converts the number, combination, and frequency of signals from the two encoders into the dfistance, direction and speed necessary to move the onscreen cursor.
6] Tapping either of the buttons atop the mouse also sends a signal to the software. Based on how many times you click, and the position of the onscreen pointer at the time of the click, the software performs the task you want to accomplish.