Aliens in movies can range from completely humanoid figures to mind-blowing, barely-perceptible-by-human-senses phantasms. Allow us to focus on the latter category, won't you? These weird aliens might tear your face off before you get a chance to truly comprehend their forms, but don't let that stop you from trying.
The Blob, "The Blob"
Sometimes simplicity can be its own kind of head-scratcher. Take this titular blob, which looks like a red lump of gelatin but is actually an all-consuming alien lifeform that eats everything it can stick to – which is everything. From its gooey first appearance latched onto the arm of some poor farmer, to its movie-theater-sized final incarnation, this is one weird alien. How does it even see??
The Body-Snatchers, "Invasion of the Body-Snatchers"
Kubrick's science fiction classic are so unfathomable that we never even see them on screen, nor can we be sure they exist. All we know is that these weird giant black rectangles keep showing up in various locales, and they might be responsible for the whole of human progress from the first time an ape used a bone as a tool to traveling all the way to Jupiter. And then there's that space baby thing … enough to make anybody's head hurt.
Alien virus, "The Andromeda Strain"
We often think of the most unfathomable aliens as being much bigger than we are, but in "The Andromeda Strain," the aliens are so small that we can't even see them. They're a bunch of little viruses, and it takes a top-secret government installation to even discover that much. God help us if anything like this ever hits Earth in real life.
The Xenomorph, "Alien"
One thing that can make an alien monster tough for we humans to fully understand is its life cycle. And the baddie from "Alien" (and its attendant sequels and spin-offs) has a doozy of one – it starts by being implanted into a host body by an equally terrifying "facehugger," before springing out of the host in a deadly form of "birth." After that it has a tendency to transform so that it looks different each time it's on the screen – and each time it kills another member of the crew.
Undersea aliens, "The Abyss"
We don't get much of a good look at the aliens in James Cameron's undersea epic "The Abyss." In fact, the longest scene involving them at all simply shows a totally-liquid "pseudopod" controlled by the off-screen aliens. The characters in the movie can't even begin to process what these aliens are really like, although they do theorize that they're somehow able to control water. Cool.
The Thing, "The Thing"
Probably the most-difficult-to-understand of any movie alien, the monster in John Carpenter's "The Thing" has existed for thousands of years, replicating everything it encountered before crashing and freezing in the snow on Earth. Now, after it wakes up, it has all those alien species stored in its memory, meaning it can morph and change into any combination of them at will. And it's pretty good at replicating humans, too. Bad news.