Mainspring barrel of a watch, disassembled, showing the mainspring.  When the owner winds the watch, the arbor at the center that the mainspring is attached to rotates, winding the spring closer around it, storing energy in the spring.  The outside of the spring is attached to the barrel and the force of the wound-up spring turns the barrel.  The ring of gear teeth on the barrel turns the watch's gear train.  The mainspring is hooked to the barrel and arbor by tabs, so it is easy to replace if it breaks.<br>  
WARNING - Do not disassemble a watch or clock containing a mainspring without the proper tools.  The mainspring contains a lot of energy and can release suddenly, causing serious injury.
HNH
CC BY-SA 3.0