This came up the other day during a rehearsal break. How do you kill a zombie? You take a shotgun and blow its brains out. What happens if you're attacked by a zombie? You become a zombie. What's the first thing a zombie does when it kills a victim? It eats the victim's brain.
So maybe the first zombie had a brain, but the new victim-zombie is brainless, because the first zombie ate it. Yet the new zombie is still staggering around, trying to kill more humans and eat their brains. You're developing generations of victim zombies here who have no brains to blow out. So how do you kill them?
Or do you only turn into a zombie if the first zombie gets distracted and doesn't eat your brain after all? Maybe the fully brain-eaten get to stay dead? Oh, the possibilities!
Okay, if the prelude to a zombie outbreak is a naked Linnea Quigley dancing in a cemetery, you might not bother with a detailed plot analysis. In fact, when it comes to horror movies in general, you probably don't want to worry too much about plausibility, whether it contains a naked Linnea or not.
For instance, does anyone think a nebbish like Seymour was really able to breed that plant? Or that anyone who looked like Jackie Joseph would want to date him, even if he did? And should we speculate as to whether Dick Miller's peculiar diet in the same film was a sly pun on Jean Kerr's 1957 book?
Do not over-analyse 'B' movies.
But still, just how do you kill a brainless zombie? The world needs to know these things, just in case.