“I’d be happy to,” Rex said. He smiled politely at the baffled woman behind the ticket desk and led Dess and Melissa into the museum.

“Psych-out,” Dess said softly. Even Melissa was smiling. Rex allowed himself a few moments of pride. At least his two friends weren’t ragging him about acrobats anymore.

The museum’s low lighting settled around them, relief from the blinding noonday sun. Rex breathed in the cool, comforting smell of exposed red clay. One wall of the museum was open to the original Bixby excavation, the walkways suspended a few feet from the raw earth. Set into the hard clay, as if never fully excavated, were tools made of bone, fossilized wooden implements, obsidian flakes in the shape of arrowheads, and the skeleton of a saber-toothed tiger. (Saber-Toothed Tiger was what the label said, anyway. Rex was certain that his own theories and Dr. Sherwood’s differed on exactly what the beast had been.)

As they headed for the sloping ramp that led down to the basement floors, Rex checked his watch. It was a few minutes past noon; Jessica might already be waiting. But on the way he paused for a quick glance into a glass case of pre-Clovis finds.

The case was full of crude arrowheads ranging from a half inch to five inches in length. Some were long and thin, others wide and barely pointed, like the end of a shovel. Most were spear points rather than true arrowheads. The makers had attached throwing shafts to them, but the wood had rotted away twelve thousand years ago. The newly arrived point was easy to spot. It was almost eight inches long, wafer thin and proportioned like a narrow leaf. It bore the telltale marks of a hammer made of soft stone and all the signs of a skilled workman. He propped up his glasses.

It dissolved into a blur; no Focus clung to it at all. Rex’s face twisted with disappointment, and he continued down the ramp. So far he hadn’t seen anything from outside Bixby that showed signs of the blue time.

In the whole world, were he and his friends really alone?

Jessica Day was already there, waiting on the lowest level, her gaze lost in a model of a mastodon hunt. Tiny Stone Age figures surrounded the elephantine animal, hurling spears into its thick hide from every direction. One of the little guys was about to be impaled on a long, twisted tusk.

“Pretty brave, huh?” Rex said.

Jessica started, as if she hadn’t heard them approach. She recovered, then shrugged.

“Actually, I was thinking twenty against one.”

“Nineteen,” said Dess. Jessica raised an eyebrow.

This is going well already, Rex thought. He’d had a whole speech planned, a regular show-and-tell. He had rehearsed it in his mind again and again before going to sleep the night before. But Jessica looked exhausted. Even with his glasses making her a bit fuzzy, her green eyes bore the marks of a sleepless night. He decided to throw out the speech.

“You must have a lot of questions,” he said.

“Yeah, I do.”

“This way.” They led Jessica to a small cluster of tables against one wall. This was where school groups ate their bag lunches. The four of them sat, Melissa pulling out her headphones, Dess leaning back precariously in her plastic chair.

“Ask away,” Rex said, folding his hands on the table.

Jessica took a deep breath, as if about to speak, but then a helpless expression came across her face. Rex could read it even with his glasses on. It was the look of someone with too many questions to know where to start. Rex forced himself to be patient as Jessica collected her thoughts.

“A hubcap?” she finally blurted out.

Rex smiled.

“Not just any hubcap,” Dess said. “That was from a 1967 Mercury.”

“Is 1967 a multiple of thirteen?” Rex asked.

“Not hardly,” Dess scoffed. “But they made hubcaps out of real steel back then. None of this aluminum crap.”

“Time-out,” Jessica called.

“Oh, sorry,” Rex said sheepishly. “Explain, Dess, but keep it simple.”

Dess pulled her necklace out of her shirtfront. A thirteen-pointed star dangled from its chain. In the dim light of the museum it caught the spotlights on the exhibits, twinkling as if with its own light.

“Remember this?”

“Yeah, I’ve noticed those all over Bixby since you told me about them.”

“Well,” Dess said, “this necklace is Darkling Protection 101. There are three things the darklings don’t like. One is steel.” She pinged the star with one fingernail. “The newer a type of metal is, the more it freaks darklings out.”

“Steel,” Jessica said quietly to herself, as if this made sense to her.

“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.”

“So all I have to do is carry a piece of metal with a thirteen-letter name around with me,” Jessica asked with disbelief in her voice, “and I’ll be fine?”

“Well, there’s a lot more to it,” Dess said. “For one thing, the metal should be clean.”