Here is the video, go watch it (it will rock your world if you don’t know your chemistry – and even if you do, it’s still great to watch). I’ll come back with a short explanation after the video.

Alkali metals are a series of metals from Mendeleev’s periodic table; you can find them right under Hydrogen, which is the first element. They include lithium, sodium, potassium, rubidium and caesium. All the metals are highly reactive under standard conditions, which is why you see all the air corrosion and the violent reactions to water. As you go down the group, the reactions become more and more violent. The typical reaction is:
Alkali metal + water → Alkali metal hydroxide + hydrogen gas