Mercury is a unique, silvery metal that stays liquid at room temperature. If you've ever seen an old-fashioned thermometer, that shiny substance inside was mercury, flowing and pooling like liquid metal from a science fiction movie. Mercury is fascinatingly dense: a gallon of it weighs over 100 pounds, so light objects like coins can float on its surface.

Scientists used mercury in thermometers and barometers because it expands and contracts predictably with temperature changes. However, mercury is also toxic, which is why most modern thermometers use safer alternatives. Breathing mercury vapor or touching it can damage the brain and nervous system, so handling mercury requires extreme caution.

The word also refers to the planet closest to the Sun, named after the Roman messenger god because it moves so quickly across the sky. Mercury the planet is a scorching, cratered world with wild temperature swings: burning hot during its long days, freezing cold at night.

In Roman mythology, Mercury was the swift-footed messenger of the gods, known for his speed and cleverness. Both the planet and the metal earned their names from this fleet-footed god.