Cesium reacting with oxygen in air. When the piece of cesiu is cut exposing a new surface of cesium atoms we see what appears to be a darkening of the sample, but the sample also seems to be melting. It is melting. As the cesium reacts with the oxygen in the atmosphere, is warms up the remaining unreacted cesium. The melting point of cesium is 28.5 just 3 degrees or so above room temperature. So it does not take much heat to cause the cesium to melt. Cool!