Quicksilver is an old-fashioned name for the metal mercury, the only metal that's liquid at room temperature. If you've ever seen a broken thermometer, you might have glimpsed those tiny silver balls that roll around impossibly fast: that's mercury, and people called it quicksilver because it moves so quickly and looks so silvery.

Watch a drop of mercury and you'll see why people thought of it as “living silver”: it seems almost alive, splitting apart and rejoining, never staying still, flowing faster than water and never getting anything wet.

The word also describes anything that changes rapidly and unpredictably. A person with a quicksilver personality might shift from happy to sad to excited in moments. A quicksilver mind moves fast, jumping from idea to idea. When something is as hard to pin down as a rolling drop of mercury, you might call it quicksilver.

Mercury itself is poisonous and dangerous to touch, which is why modern thermometers use safer materials. But the word quicksilver remains useful for describing anything that moves with that same restless, shimmering, impossible-to-catch quality.