Deuterium

I spent a lot of after-school time watching Hogan's Heroes as I was growing up. Sargeant Schultz was always good for a laugh, and I can remember his references to "heavy vasser" and a red bucket that was labelled so. Many years later I learned that heavy water was of some concern during the war, as the Germans appeared to be experimenting with it as a moderator for nuclear reactions. To the allies, this indicated an interest in developing nuclear weapons. Heavy water is water containing deuterium (D) the isotope of hydrogen that contains a neutron and hence that has an atomic mass of 2 rather than one. So heavy water weighs 20 amu/molecule, or 20 grams per mole: a modest 10% more than regular water.

Disrupting the development included a really dramatic destruction of a supply of heavy water, which was all produced during that time in Norway. The PBS series Nova has described both the sinking of a ferry carrying the stuff on "Hitler's Sunken Secret," as well as an attack on the plant that manufactured the stuff in the first place. The plant made use of a series of multiple distillations, if I recall correctly, to enrich samples of water, which naturally contains a small amount of D2O.

Gilbert Lewis (of Lewis structure fame) was the first person to isolate D2O, incidentally.

Heavy water is actually a little more dense than regular water, since deuterium weighs a little more than hydrogen. But the effect is fairly modest and wouldn't be noticeable if you were carrying a little flask of the stuff.