“Basically, darklings are really old,” Dess explained. “And like a lot of old people, they don’t like stuff that’s changed since they were born.”

“They used to be afraid of cut stone,” Rex said. “Then forged metals: bronze and iron. But gradually they got used to them. Steel is newer.”

“Hasn’t steel been around a long time?” Jessica asked. “Like swords and stuff?”

“Yeah, but we’re talking stainless steel, a modern invention,” Dess said. “Of course, one day I’d like to get my hands on some electrolytic titanium or—”

“Okay,” Jessica interrupted. “So they don’t like new metals.”

“Especially alloys,” Dess said, “which means a mix of metals. Gold and silver are elements. They come straight up from the ground. The darklings aren’t scared of them at all.”

“But they’re scared of alloys. So they couldn’t get through something made of steel?” Jessica asked.

“It’s not that simple,” Dess said. “Thing number two that darklings are afraid of is…math.”

“Math?”

“Well, a certain kind of math,” Dess explained. “There are certain numbers and patterns and ratios that freak them out, basically.”

Jessica’s expression remained one of disbelief.

Rex had prepared for this. “Jess, have you heard of epilepsy?”

“Uh, sure. It’s a disease, right? You fall down and start foaming at the mouth.”

“And bite your own tongue off,” added Dess.

“It’s a brain thing,” Rex said. “The seizures are usually triggered by a blinking light.”

“It doesn’t matter how strong or fit you are,” Dess said. “A blinking light and you’re suddenly helpless. Like Superman and kryptonite. But the thing is, the light has to be flashing at a certain speed. Numbers work that way on the darklings.”

“And that’s why Bixby has this thing about thirteen?” Jessica asked.

“You got it. Guaranteed protection against darklings and their little friends. Something about that number drives them totally crazy. They can’t stand symbols that mean thirteen or groups of thirteen things. Even thirteen-letter words fry their heads.”

Jessica let out a low whistle. “Psychosomatic.”

“Yeah, that’s a good one,” Dess said. “So I gave that old hubcap a thirteen-letter name, Hypochondriac, and psychokitty got burned.”

“Sure,” Jessica said.

“Just remember to always keep a fresh tridecalogism in your mind.”

“A fresh what?”

“Tridecalogism is a thirteen-letter word that means ‘thirteen-letter word,’” Dess said, grinning happily.

“Really?”

“Well, I kind of made it up myself. So don’t try to use it to protect yourself. And remember, when you actually use a tridecalogism on a darkling, make sure you come up with a fresh one for the next night.”

“They get used to words faster than they do metals,” Rex said.

“Who knows?” Dess continued. “Maybe one day they’ll get used to the number thirteen. Then we’ll be looking for thirty-nine-letter words.”

Rex flinched at the idea. “That’s not going to happen anytime soon.”