There is this urban legend that has been circulating for years, possibly decades.

It seems there was once this inventor who came up with a revolutionary automobile engine, an engine that could change everything. This remarkable feat of engineering, according to the story, ran on nothing more exotic than plain, ordinary, everyday water. H2O.

But before the inventor could go public, he disappeared, and his invention vanished along with him. Their whereabouts remain unknown to this day.

Meanwhile, still lurking in the shadows is Big Oil, the prime suspect in the inventor's disappearance, still raking in profits from our dependence on gasoline.

It's a nice fable, and it might frighten children at bedtime, but you'd have to have your own talk show on late night radio to be stupid enough to believe it, or to believe any of the other "water engine" stories floating about.

David Mamet, one of the better screenwriters working today, wrote a radio play about the legend, which he titled, appropriately enough, "The Water Engine." The radio drama begat a stage production, which in turn begat a television movie. So as you can see, the water engine has gotten a lot of mileage, metaphorically speaking, of course.

It is no surprise then that the water engine resurfaced on Sunday's installment of "The Lone Gunmen," the "X-Files" spin-off series about three conspiracy-obsessed geeks -- Langly, Frohike and Byers -- taking on the forces of the military-industrial complex in a never-ending battle for Truth, Justice and the American Way.

Starring Dean Haglund, Tom Braidwood and Bruce Harwood, "The Lone Gunmen" takes a more earthbound approach to conspiracies than does the show that spawned it. Rather than dealing with alien bounty hunters and plague-carrying bees like their pals Mulder and Scully, the Gunmen go after corrupt corporations and rogue elements of the U.S. government.

So far the results have been mixed. The first two episodes were written by committee, and it shows. They are a hodgepodge of unfunny gags and lame plot twists. The second two episodes, while nothing spectacular, show progress and are helped by being the products of one scriptwriter each.

Assuming "The Lone Gunmen" can avoid cancellation, it may eventually become a good TV show. But it probably won't get the chance. The show's ratings are in a free fall of nearly XFL-like proportions. Since its debut, "The Lone Gunmen" has lost more than one-third of its audience.

On the up side, the fewer people who watch "The Lone Gunman," the fewer people who might be tempted to believe in things like engines that can run on water.

Biologist and skeptic Richard Dawkins has worried that "The X-Files" encourages people to believe in absurd things, like extraterrestrials coming to Earth to make pretty designs in our wheat fields. I tend to doubt that. "The X-Files" is obviously science fiction, and while it may give unwarranted solace to True Believers, I don't think it creates new ones.

"The Lone Gunmen" on the other hand, despite its slapstick trappings, is just earthly enough to seem believable.

It's one thing to believe in something that is obviously crazy, like aliens travelling billions of miles just to stick probes where the sun doesn't shine. It's quite another to believe in a mundane conspiracy, like Big Oil suppressing a new technology that threatens its profits.

Fortunately, it only takes a little effort to reveal the water engine for the urban myth it is.

Such an engine could never work, because it would have to break the Second Law of Thermodynamics: It is impossible for an unaided self-acting machine to convey heat from one body to another at higher temperature.

In water, hydrogen atoms are bound to oxygen atoms. To work, a water engine would have to split off the hydrogen atoms and burn them for heat. But burning is simply a rapid form of oxidation, meaning the hydrogen atoms would recombine with the oxygen atoms to give off -- you guessed it -- water. Even assuming no energy loss (which is impossible), you could only get as much heat energy out of the hydrogen as you put into getting the hydrogen in the first place. (Because, according to the First Law, energy can neither be created nor destroyed.) There would be no energy left over to move the vehicle.

Never mind that you wouldn't want an engine that gave off water vapor as exhaust, anyway. Imagine the humidity in a city where all the cars spewed out steam. Trust me, you're better off breathing CO2.