A mainspring is the tightly coiled spring inside a mechanical watch or clock that stores energy and powers the device. When you wind a watch, you're twisting this spring tighter and tighter. As it slowly unwinds, it releases energy bit by bit, turning the gears that move the hands around the clock face. Before batteries and electricity, mainsprings were essential for keeping accurate time.

The mainspring sits at the heart of the mechanism, which is why the word also means the most important reason something happens or the driving force behind it. If curiosity is the mainspring of scientific discovery, that means curiosity is what powers and motivates scientists to investigate the world. When a teacher says that practice is the mainspring of improvement in math, she means practice is the central force that makes students better. Like the coiled spring in a watch, the mainspring of any activity provides the energy and motivation to keep everything moving forward.