A pendulum is a weight suspended from a fixed point that swings back and forth in a steady, regular rhythm. You've probably seen a pendulum if you've ever watched a grandfather clock: that brass disc swinging beneath the clock face helps the clock keep time.

The motion of a pendulum is remarkably predictable. Galileo studied this in the early 1600s, supposedly after watching a lamp swing in a cathedral. He noticed that when the swings were small, it took almost the same amount of time to complete each swing, even if the lamp swung in slightly wider or narrower arcs. This discovery helped lead to pendulums being used in clocks for over 300 years, making them among the most accurate timekeepers humans had ever built.

Scientists use pendulums to study motion and gravity. A Foucault pendulum is a massive pendulum that swings for hours, slowly changing direction as the Earth rotates beneath it, a beautiful demonstration that our planet is actually spinning.

The word also describes situations that swing between extremes. When fashion swings like a pendulum from baggy clothes to fitted ones and back again, or when political opinions shift from one extreme to another, people invoke the image of that steady back-and-forth motion. What goes one way eventually swings back, just as reliably as a clock's pendulum marking the seconds.