Cesium is among the most active metals on Earth — only francium is more so, but it’s also radioactive, which cesium is not. Cesium has a very low melting point of 29°C (84°F), and it melts even while you hold a vial with it in your hands. Otherwise, when left alone, the metal hardens, forming very beautiful crystals of pure cesium. It also has lots of applications, one of which is powering atomic clocks — the most precise ones in the world.