So when a creature flick comes along that is so preposterously ridiculous, so patently absurd, so monstrously unscientific ... well, we don't actually notice until years later when we need a premise for an article. But then we have no choice but to feel betrayed and scream "BULLSHIT!" at the top of our lungs.
As seen in: Alien
The Facehugger was just one of several horrific stages in the life cycle of the alien race in Alien. Basically, the thing was an octopuss-looking abortion of squirming slime, whose whole purpose was to implant the next phase of itself into a host. And it did so by facerape.
It latched on to a guy's mouth and pumped its offspring down his throat, which later burst out of the poor fool's chest.
Why We Call Bullshit:
What do aliens eat? Do the movies ever establish that? Well, they'd better eat the exact same damned diet as humans, since their offspring apparently have to survive on the exact same nutrients found in a human body. After all, a human embryo/fetus requires an incredibly specific set of nutrients, so much so that artificial wombs are still science fiction. And keep in mind, this species didn't evolve specifically to use humans as hosts--as far as we know, they had never encountered humans before the events of the first film.
Alien tries to bullshit its way around this by saying that the Alien's DNA merges with the host; this becomes totally ridiculous when you realize that, despite sharing a good 99.9 percent of our DNA with chimps, we're still genetically incompatible with the fuckers. Since the Alien is way, way less like us than the chimp, this would be like saying a Komodo Dragon could successfully impregnate a human.
But even that analogy breaks down when you realize that the Alien was not only completely different from humans, it was completely different from every living creature on the planet. Yes, unlike carbon-based life (a.k.a. fucking everything), the Alien was a silicon-based creature: At one point in the movie, we see it replacing its exoskeleton with polarized silicon, which would be like if you replaced your skin with glass.
To update our comparison, the Facehugger's reproductive method would be like if your window came to life and made passionate love to you, and then a week later you gave birth to shot glasses.