|My grandmother used to have this watch she wound by shaking it. How is it possible?
Her watch was called an automatic watch and contained a heavy piece of metal at the end of a tipping arm. As her wrist moved during daily activity, the metal piece would swing back and forth, twisting the arm first in one direction and then in the other. The arm's twisting motion would wind the watch's mainspring, just as you can wind the mainspring of a normal wristwatch by rolling the knob back and forth in your fingers. On some automatic watches, you can feel and hear the weight swinging back and forth as you shake the watch.