Cesium, or caesium, is an alkali metal on the periodic table with the atomic number 55. It is highly reactive and pyrophoric, meaning that the mere oxygen in the atmosphere is sufficient to make it go up in flames.
With an extremely low melting point of 28.5 degrees Celsius, or about 83 degrees Fahrenheit, cesium is one of only five elemental metals that is liquid at near-room temperatures. Per gram, cesium is more expensive than gold, and when it solidifies, it forms delicate crystal structures that even look like gold.
But toss it in water, or just leave it exposed to the ambient air, and it will self-ignite to send up purplish-pink chemical flames. Even if the stuff is cooled to minus 177 degrees Fahrenheit, dropping it in water will cause an explosive reaction with the oxygen in the liquid. You can see it in action in this video from Thoisoi2 – Chemical Experiments!, which demonstrates some of the volatile reactions that cesium will produce. (Mixing it with molten sulfur is particularly explosive.)
But cesium isn’t all explosions and colorful fire. In 1967, measurements of the emission spectrum of cesium were used to officially define both the amount of time in a second and the length of a meter. Today, the most accurate atomic clocks in the world—which only deviate by a single second every 100 million years—use atomic cesium to measure time. The reactive element is also used as a propellant in some ion thrusters found on satellites.
In the right hands, this nasty stuff can be incredibly useful. Just make sure not to mix with with, well, pretty much anything.