Aside from their boring, nonaggressive brethren who come to Earth simply to observe us — and, perhaps, probe us — extraterrestrials usually make a spectacle of themselves.

This is certainly the case in Steven Spielberg's pyrotechnic blockbuster "War of the Worlds," starring Tom Cruise as a deadbeat dad who leaps into action to save his children from a full-scale alien invasion.
But why do aliens come in the first place? The reasons are many, neatly reflecting our own attitudes and histories regarding exploration and primitive cultures. Let the probing begin.

THEY WANT OUR LAND

However spoiled by smog, toxic waste and nonbiodegradable nappies, Earth is still one prime piece of intergalactic real estate. Why else would aliens be so eager to wrap their tentacles around this remote little interstellar backwater?

We never find out what the city-pulverizing squid-people of "Independence Day" had in store for our planet (a chain of day spas, maybe?), but in Spielberg's "War of the Worlds," the invaders unleash a crimson-colored form of plant life to make Earth more like home. Their home.

Director David Twohy played around with a similar idea in "The Arrival" (1996), envisioning alien interlopers who purposefully speed up global warming to make the planet more hospitable. They're called Hummer salesmen.

THEY WANT OUR WOMEN

One thing is certain: Earth's females are the galaxy's finest. So fine, in fact, that aliens routinely travel enormous distances to mate with them. The 1967 sci-fi turkey "Mars Needs Women" is fairly clear on this point.

Other extra-terrestrial gigalos prefer to wade in our gene pool anonymously. In "Village of the Damned" (1960) — later remade by director John Carpenter ("The Thing") — a quaint English village is scandalized when every female of child-bearing age simultaneously finds herself pregnant. Nine months later, each of them gives birth to a creepy tow-headed child with powers of mind control. The lesson: Aliens are definitely down with O.P.P.

"Species" (1995) featured a similar premise, albeit with the genders flipped. A drop-dead gorgeous, half-alien tramp (Natasha Henstridge) cuts a bloody swath across Los Angeles looking for a human male with whom to propigate her species. Finding a donor proves unnecessarily problematic for Henstridge (Hello? Sperm bank?), but it could have been worse. They could have genetically-engineered her to look like Rosie O'Donnell.

THEY WANT OUR JOBS

Reflecting the social anxieties of the time, the wayward extraterrestrials in "Alien Nation" (1988) came to Earth not to conquer us, but to seek political refuge, much as the Mariel boatlift brought scores of Cuban émigrés to our shores.

In John Carpenter's "They Live" (1988) the alien invasion is of the white-collar variety, illustrated in the story of an out-of-work construction worker (Roddy Piper) who uncovers a conspiracy that has promoted hideous, goo-faced aliens at the top levels of government and business.
Nonetheless, "They Live" has been dubbed a work of science "fiction."

THEY WANT TO EAT US

Beware aliens professing a desire to "serve man" — as the classic "Twilight Zone" episode demonstrated, this could be code for an exotic dining experience you won't soon forget.

Other aliens have found humanity delightful to the taste buds. In the NBC miniseries "V" (1983), a wave of human-looking aliens wearing snappy red jumpsuits pull up in a fleet of enormous space-saucers, bearing tidings of peace. In reality, they're just lizards who want our man-meat.

THEY WANT TO ENLIGHTEN US

Aha, now we come to that most insidious of alien xenotypes: The spacemen who proselytize. Does anybody really think that Richard Dreyfuss is up there with his hairless pink-skinned pals from "Close Encounters of the Third Kind" (1977) having a good time? It's missionary work and encounter groups from morning 'til night, and who knows how many issues of "Dianetics" that poor man has had to buy.

Other aliens have been even more overtly paternal. In Robert Wise's cautionary classic "The Day the Earth Stood Still" (1951), an extraterrestrial messenger named Klaatu comes to Earth with a stern warning to make peace, or else. Klaatu's profession: Carpentry. No wonder the movie continues to inspire religious devotion among fans.