Hmmm I coulda sworn that anything in the second column of the periodic table exploded when it came in contact with water (well, almost everyting) and that is Sodium and Potassium included. If you get a pretty decent size chuck of Sodium, it will blow up. Some kids blew up a toilet at my school a few years ago, almost killed them. And your chem teacher, he didn't take the stuff out of water, Potassium and Sodium are stored in oils, i'm almost positive on that, and if you aren't careful, the oils can dry up over time and lead to some very dangerous situations.