For practical purposes, the metal Cesium can be found in spacecraft propellant, radiation devices that treat cancer and atomic clocks. For impractical purposes, it reshapes itself and explodes in the most beautiful and unpredictable ways.
Cesium (also spelled Caesium) is one of the most extreme elements on the periodic table -- depending on how you're measuring, it's the most reactive and the softest metal. It's explosive in water as cold as −177 °F (−116 °C) and it spontaneously ignites in air. Its limited supply means that its price is, on average, higher than gold. But none of that's why we're here.
We want to watch this shit burst into a small magenta flame and spread scorched tendrils across a piece of wood. We want to see tiny fragments explode when tossed at a graffiti-covered wall. And above all, we want to see it form and un-form lovely metallic snowflakes as its temperature rises and falls.
All of that and more, is in this video by Thoisoi: