Dihydrogen monoxide is the name given to a "colorless and odorless chemical compound" that is pervasive in the environment. It's also a fancy-pants way of saying water.
In 1997, the DHMO hoax was revived by 14-year-old Nathan Zohner who brought it to the public's attention. Zohner petitioned to get DHMO banned as the basis of his science project, titled "How Gullible Are We?" Many people were very gullible, and this hoax occasionally comes back to haunt politicians.
MP tries to ban water at The New Zealand Herald for an example of an official who fell for the hoax.
Dihydrogen Monoxide at Snopes for the email that was circulated on this subject.