Butters looked at me blankly. "Which means what?"

I leaned a hip on the counter and explained. "A threshold is a kind of energy that surrounds a home. It's…" I frowned, thinking how to explain it. "It's sort of like the home has a positive charge to it. If outside magic wants to come in, it has to neutralize that charge first. Big, tough things from the Nevernever need a lot of power just to stay in our world. They don't usually have enough to take out a threshold and still have enough juice to be dangerous."

"It's like that vampire thing?" he asked. "They can't come in if you don't invite them?"

"Pretty much, yeah. If you invite something in, your threshold won't affect it. But other magical beings and energy have trouble with it. It's a solid defense."

"Didn't help your place much," Butters observed.

"My place is a rental apartment," I said. "And except for the past several months, it's been just me living there. Doesn't give it the same kind of energy as you'd find in a long-established home."

"Oh. Is that what they mean by 'safe as houses,' then?"

I smiled a little. "A house doesn't make a home. When the place has got history, family, emotions, worries, joys worked into the wood, that's when it gets a solid threshold. This house has been in the Murphy clan for better than a hundred years, and lived in for every one of them. It's solid. You'll be safe in here."

"But it's not going to get loose once you call it up," Butters said. "Right?"

"That's the plan. But even if it did, you aren't the one who is going to piss it off. There won't be any reason for it to come after you."

"Oh, good," he said. He blinked at me and said apologetically, "Not that I want it to come after you, Harry."

"I don't blame you," I said.

Butters nodded. "Why zombies?" he asked.

"Huh?"

"Sorry. Changing topics. New question. Why do all these necromancer types use zombies?"

"Not all of them do," I pointed out. "Corpsetaker had called up a bunch of semicorporeal ghosts. Specters."

"But human," Butters said. "Zombies look human. Specters look human. Why not whistle up a pack of decayed rats? Or maybe semicorporeal mosquitos? Why use people?"

"Oh," I said. "It's got to do with a kind of metaphysical impression that any given creature leaves upon its death. Sort of like a footprint. Human beings leave larger footprints than most animals, which means that you can pour more energy into reanimating them."

"They make stronger goons," Butters clarified.

"Yes."

"How come Grevane had fresh corpses when he came to get me, but he attacked your house with old ones? I mean, I saw those things up close." He shivered. "Some of them must have dated back to the beginning of the twentieth century."