The metals in Group 1 of the periodic table react vigorously with water. These elements include lithium, sodium, potassium, rubidium, and caesium. This is why, if these metals are purchased for a chemistry lab, they are stored in dry mineral oil. You may have seen videos that people have taken as they toss a chunk of sodium metal into a lake or body of water. The reactivity increases as you go down the table toward caesium. 

The reaction proceeds as follows (I'll use sodium in the example, but it could be any of the Group 1 metals):

2Na(s) + 2H2O(l) —> 2NaOH(aq) + H2(g)

As you can see above, the gas formed during the reaction is hydrogen gas. In more explosive reactions (with elements such as potassium, rubidium, and caesium), enough heat may be created that the hydrogen gas catches fire. In fact, caesium explodes when exposed to water.