Extra-terrestrials are weird. Sometimes, really weird. They may look relatively normal or mind-bendingly freakish, but whatever their appearance, you can bet that they don't work like us. They may have green blood or six sexes or any of a variety of other features that make it clear: these are alien! Science Fiction at the hard end of the Mohs Scale of Science Fiction Hardness is more likely to feature really bizarre examples of Bizarre Alien Biology, though the soft end can get pretty weird at times too, especially when the Rule of Cool or Rule of Funny is in play. Compare Anatomy Tropes.