Cesium reacting with water. Here the piece of cesium sinks to the bottom of the beaker as its density is greater than that of water. Nothing happens immediately because the liquid the cesium is stored in is still surrounding the piece of metal. It takes a moment for that liquid to be washed of the piece of cesium. As soon as it is we begin to see large bubbles of hydrogen gas produced. As more of the liquid is removed the remaining smaple is exposed, and the last of the unreacted cesium goes off all at once exploding out of the water.