Explain how a mechanical watch works

A simple mechanical watch works by transferring power from the main spring, a tightly coiled metal spring, through an escapement and balance wheel, to wheels representing seconds, minutes, and hours.

The watch uses a gearing from the main spring that pushes an escapement gearing against a ruby palette to oscillate a balance wheel. The balance wheel and escapement are carefully constructed to oscillate at a specific frequency, releasing small bursts of power to the gear train. The train is constructed to gear the specific frequency of the balance wheel into seconds, minutes and hours. 

Further complications like chronographs, sun phase, alarms, and others are not accounted for in this simple watch. An automatic watch contains a weighted rotor in order to wind the mainspring with the motion of a person's arm.