This something I’ve found interesting, ever since little I always thought watches were made first using batteries but it is the opposite and its very intriguing to know that it is possible by using a simple spring.
This is an image of the main spring used in current automatic watches.
Automatic watches requires winding on the knob which is also used to set the time, to give the watch energy to move all the components inside. Of course over a period of time the watch will eventually stop moving (estimated around 2 days) but it will not if its worn because it uses the persons movement to move the spring within the watch to keep the watch working.
Other methods have also been tested such as using solar energy to power up the battery, however it is simply not as clever to create automatic movements inside a watch.