Cesium is very rare. However, if one could find a lump of solid cesium and throw it in water, it would certainly make an entertaining explosion. We can predict this due to cesium’s position on the table. It is in the alkali metal family, and therefore reacts similarly to other alkali metals. Alkali metals react with water according to the equation:
M (s) + H2O –> MOH (aq) + H2 (aq)
Where M is any alkali metal. This rxn usually forms hot H2 (g), which explodes in air due to its reaction with oxygen:
The reaction between alkali metals and water occur with increasing vigor as one travels down the group. Because cesium is quite low on the periodic table, we’d predict it would have a very vigorous reaction with water.