I woke up the other day wondering why vampires in movies sometimes have longer teeth, sometimes shorter.    Consider, the Cullen clan from the Twilight saga have fangs that are barely longer than normal human teeth,  whereas the blood-suckers from the Underworld movies have really long canines.  Other movies continue the trend. The original Dracula?  Long fangs.  Dark Shadows?  Medium fangs. There seems to be no standard length for vampires.  Even within the movies there is variation.  In Underworld,  the lead character's rival has significantly longer teeth than she does.  There must be some explanation.  Now all these vampires share the same diet, and, considering the whiteness of the teeth, all attend to oral hygiene.  So what is the difference?  Genetics doesn't seem to offer much of a lead;  how do vampire genes even get passed along?  By bites?  Then I thought maybe there was some sort of baby-teeth/ adult-teeth thing going on,  but we never see them trying to wriggle out a loose tooth, hoping the tooth fairy will visit their coffin.  The answer did not occur to me until I saw a friend's pet gerbil.  These pets, along with many other mammals, constantly gnaw to keep their continuously-growing teeth from getting too long.  This must be the explanation for vampires.  Those canines of theirs must be constantly getting longer, and different movies catch them in various stages of growth.  Having an all-liquid diet, the vampires must be chewing on wood when humans aren't around.  This explains why so many vampires live in the forest.  It also makes sense that humans are always killing vampires with wooden stakes: there must be dozens of sharpened sticks lying about.  And perhaps that's why vampires leaves their coffins; they've chewed a hole right through it and need to find a new one.