Explosive water tricks
Two videos showing what happens when alkali metals encounter water:
(Thanks to Charles D and BDan for the links.)
Added later:
J7y points out that, unfortunately and disappointingly, the Brainiac video (the second one I linked to above) was faked. (See followup article for confirmation.) He notes that Alkali Metal Bangs provides some legitimate info about this kind of thing, plus some non-faked videos.