Abhors a vacuum
Astroprof has one of the most interesting blog entries I've read in a while: What happens when the human body is exposed to a vacuum.
Which immediately brings to mind that cartoonishly yucky scene in Total Recall where the governator starts to experience explosive decompression on the surface of Mars. And, for those of you who were into Farscape, the fact that one of their alien characters (d'Argo) could survive for several minutes in a vacuum. They never explained how, exactly, but it was a crucial plot point in at least two episodes.
Image from farscapeworld.com