I once saw an episode of the Open University in which the demonstrated the reactivity of the alkali metals.
First they dropped lithium in the water. - it fizzed and bubbled a little like an alka-seltzer.
Then they dropped in sodium & potassium which caught fire much as everyone else has said.
That's where the experiments usually stop in schools cause they have already demonstrated that the alkali metals get more reactive as they go down the series. The OU however hadn't had enough yet. So they got some rubidium out and put that in the water. The flames were even more impressive and IIRC the water even started to boil close to where the metal went in.
At this point most sane people would stop but the OU decided to get out some Cesium. At this point is worth noting that the person doing the experiment was no longer visible on camera. Instead you saw a pair of long handled tongs appear on camera. The second the cesium hit the water there was a loud bang and the water trough shattered.
I think that was when I decided I wanted to study chemisty
