The movement is the heart of a watch, quite literally. It is motion that makes a watch tick, keeping time and powering its functions. But where does that motion come from?

In the early days of clockmaking, the motion was driven by a pendulum. The potential energy stored by winding the clock was released into the clock’s movement by a pendulum. The pendulum swings back and forth with a regular, constant speed that releases energy in measurable increments, powering the time keeping of a watch. However, pendulums are only reliable when a clock is still; they don’t work well in portable devices.

Eventually, the watch was invented. All early watches were “mechanical,” as we would call them today. In mechanical watches, the motion is driven by the unwinding of a spring, called a mainspring. The watch is manually wound, and then the spring unwinds in regular, measured increments that drive the timekeeping of the watch. Mechanical watches need to be wound every day, as the mainspring slowly unwinds over the course of 24 hours.

Later still, automatic watch movement was invented. In an automatic (or self-winding) watch, the mainspring is wound by the natural kinetic motion of a person’s body as they move. Specifically, an automatic watch has small round weights called “rotors” inside. These rotors are allowed to spin freely, reacting to your natural movement as you walk, run, gesture, or use your watch-side hand in any way. The spinning of these rotors transfers rotational movement to the mainspring, which is wound accordingly, and then unwinds and drives the watch.

Automatic watches tend to be larger and heavier than manual mechanical watches, because the case has to contain the rotors that are unnecessary in manual watches.

However, it’s a mistake to think that automatic watches never need to be manually wound. Many people don’t wear the same watch every day, or take their watch off for various reasons. When the watch isn’t being worn, the rotors can’t wind the mainspring, and the watch stops.

Most automatic watches have something called a “power reserve,” which simply indicates the amount of time that the watch will continue to operate while it isn’t in motion. In a high-quality automatic watch, it may continue to function and keep time for a day or two without being worn. In a low-quality automatic watch, it may stop keeping time overnight, while you aren’t wearing it (or are wearing it, but your body is still). In these cases, manually winding and re-setting it every morning isn’t usually a huge chore, but then, if you were going to manually wind and set your watch every morning, why not simply get a mechanical watch to begin with?

Many mechanical watch owners actually enjoy the daily ritual of winding their watches, and find it meditative. And, generally speaking, manual watches are more durable and easier to repair, because they have fewer moving parts. But automatic watches are much more convenient for daily wear, and more reliable, since winding your watch is one less thing to remember.