heavy water

Heavy water is H₂O, just like plain old water. It is `heavy' because each of the hydrogen atoms has an additional neutron. The additional neutrons do not affect the chemical properties of heavy water, but they do give it a greater molecular mass, and cause it to block stray neutrons better (hence its use as a moderator).

Heavy water gains its name from the isotope deuterium replacing normal hydrogen. Heavy water is fairly rare in nature, and costly to refine - an ice cube would cost roughly $100. However, the added mass of the extra neutron would make the ice cube sink in normal water, an interesting if expensive science experiment. Heavy water is an important part of nuclear weapon research. During World War II, certain lakes in Northern Europe with a greater than normal ratio of heavy to normal water were important military objectives for both the Allies and the Axis.

Heavy water's isotopic nature also makes it fatal in large amounts. Although deuterium, like all isotopes, has nearly identical chemical reactions to the non-isotopic hydrogen, the extra mass retards chemical reaction rates. With enough heavy water over a very long period, the chemical reactions of one's body would slow down to the point of death. Xunker has mentioned that it would interfere with mitosis, specifically. No clue if this has actually been done before, and it'd be one of the most expensive poisonings ever.