Sodium and water
As schoolboys, many years ago, we found putting a pellet of sodium into water one of the more entertaining moments in the study of chemistry. Theoretically, you could pinch a larger lump from the lab and, after school, put it in a coca cola and retire a safe distance to watch it explode. Purely theoretically of course...
In 1947, these guys had the chance to do the experiment on a larger scale:
In 1947, these guys had the chance to do the experiment on a larger scale: