A small amount of Cesium metal. Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it liquid at or near room temperature (as seen from the gif).
It is highly reactive and very pyrophoric, it ignites spontaneously in air and reacts explosively with water. Because of it’s high reactivity caesium can be stored in vacuum-sealed borosilicate glass ampoules.
Interesting fact: In 1860, Robert Bunsen and Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Due to the bright blue lines in its emission spectrum, they chose a name derived from the Latin word caesius, meaning sky-blue. Caesium was the first element to be discovered spectroscopically, only one year after the invention of the spectroscope by Bunsen and Kirchhoff.