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A self-winding watch is simply a watch that winds itself. More specifically, it is a mechanical watch that uses the natural movement of your arm and gravity to power itself. No batteries or manual winding is necessary.

Typically, the watch will have a small weight attached to a gear. On my watch, the weight is a semi-circular disc attached to a central pivot. The weight will pivot with each small movement, winding a spring.

Of course, if you leave your watch off for long enough, it won't be able to wind itself, and it will stop. Most watches can store up enough energy for a day or two.