"Mm," Fierenzo said, taking another sip of his water. "Let me see that gun again, will you?"

Jonah's forehead wrinkled, but he set his own water bottle aside and held out his left hand. "Here's what it looks like sheathed," he said, pushing up his jacket sleeve to reveal an elaborately decorated metal wristband. "I twist my wrist so to throw it—"

He turned the wrist sharply over, and Fierenzo watched in fascination as silvery tendrils shot out of the wristband's underside, flowing up along the insides of Jonah's fingers and thumb and then bending and curving around each other like a mutant pretzel before melting together into the now familiar flattened cylinder shape. "And there it is."

"Yes," Fierenzo said, nodding. Now that he had a clear look, he saw something he hadn't noticed before: where the wide wristband had been only a slender loop of wire remained, encircling Jonah's wrist and attached to the grip of the hammergun by an equally thin metal wire. "Is that loop supposed to keep you from dropping it?"

"It does that, too, but it's mostly there to give the hammergun a path to flow back along when you sheathe it," Jonah told him. "Like so."

He opened his hand and the hammergun went into reverse, untwisting itself and flowing back along the fingers to re-create the original wristband. "That is truly amazing," Fierenzo said, shaking his head. "How exactly does it work? It doesn't fire slugs, does it?"

"In a way it does," Jonah said, flipping his wrist and bringing the weapon out into his hand again. "It fires small force bubbles that accelerate away—"

"Wait a second," Fierenzo interrupted. "It fires what?"

"Force bubbles," Jonah repeated. "Little spheres or disks of non-solid force. A bubble accelerates away from the muzzle, growing bigger and gaining speed along the way, until it runs into a solid object. At that point it dissipates, transferring its energy and momentum to the target."

"How many settings are there?"

"Just the two: ball or disk," Jonah said. "The ball is spherical and delivers its energy like a hammer, while the disk has an edge to it and is more suitable for cutting."

"So the ball is what you hit the Greens with yesterday night by the station house," Fierenzo said slowly, trying to sort this out. "While the disk is what that other Gray cut off the tree branch with over in Yorkville?"

"Yes to the first; I don't know to the second," Jonah said. "Depending how far away the tree was, either setting could probably have taken off a branch."

"Wait a minute," Fierenzo said, pressing his fingertips to his forehead. "I thought you said you hadn't killed the two last night."

Jonah sighed. "Watch," he said. Sticking his free hand directly in front of the hammergun muzzle, he squeezed the trigger.

There was a faint pop, but as far as Fierenzo could see nothing else happened. "Like I said, the shot picks up energy and momentum as it travels," Jonah said. "Right up close—" he fired into his palm again "—nothing much happens. A little farther away—"

Taking his hand away, he fired at one of the empty water bottles a foot away from him. The shot sent it skittering across the concrete. "—and you can start to feel it," he said. "Farther away yet—" he lifted the weapon to aim at an imaginary horizon "—and you could theoretically crack off a piece of a mountain."

Fierenzo shook his head. "Sounds pretty damn dangerous. I've never even heard of a weapon that doesn't work up close."

"That's probably because hammerguns aren't technically weapons," Jonah said. "They were designed as mining and stoneworking tools. Low-power at close range for delicate shaping; high-power farther away to give a kick to the ore vein or surface formation you're working."

"Uh-huh," Fierenzo said as something suddenly occurred to him. "Which means that when you shoved that thing in my face on the fire escape it was a hundred-percent bluff."

"Basically," Jonah admitted. "But what else could I do?"

"I suppose," Fierenzo conceded. "And that's why the second Green last night ran toward you instead of away. He was trying to move in to where you couldn't hit him as hard."

"That, plus the fact that the sonic nature of the Shriek means it gets stronger as you get closer to it,"

Jonah said.

"Tell me about it," Fierenzo said ruefully, rubbing his ear gingerly. "Was that Jordan up on the building playing target?"

Jonah nodded. "He was trying to hold their attention so I could get behind them." He grimaced.

"Though if I'd realized how strong the Shriek could be even at that distance, I'd never have let him do it. I guess I've never seen a Warrior in action before."

"So the Greens will always want to fight up close, while the Grays will always want to fight at a distance," Fierenzo concluded. "That'll make for some interesting battlefield tactics. Where exactly do these force bubbles get their energy?"

"They draw heat from the air molecules along their path and convert it into kinetic energy," Jonah explained. "That's why you usually see a white line, at least if the bubble's traveled far enough.

That's frost that forms where the air's suddenly had some of the energy sucked out of it and gone cold. Sometimes you can even get snowflakes drifting off the line."

"Sounds very festive," Fierenzo said. "What do you use to hang onto buildings?"

"Nothing but natural talent," Jonah said. "It has to do with van der Waals forces between our bodies and the metal in the walls, or some such thing. I'm a little vague on the details."

"Close enough," Fierenzo said. Physics had never been his strong suit, either. "Now, what about this tension line thing you mentioned earlier? How does that work?"

Jonah made a face. "What do you want, Fierenzo, a short course in Gray tech?"

"Humor me," Fierenzo said. "You're trying to convince me to keep my mouth shut, remember?"

"And there's no point in keeping quiet about one secret when you can have a hundred secrets to keep quiet about instead?"

"Exactly," Fierenzo said. "Come on, give."

With an exasperated sigh, Jonah reached beneath his coat to the side of his belt and pulled out a device about the size of a cigarette pack but flatter. "Fine," he said, tossing it into Fierenzo's lap.

"There it is. Go ahead—figure it out."

He leaned back against the wall, folding his arms across his chest. Picking up the device, Fierenzo gave it a quick study. It looked something like a tailless manta ray, with one side flat and the other smoothly curved, and seemed to be made of the same metal as Jonah's wristband. In the front, where the manta's mouth would have been, there was a finger-sized ring connected to a slender thread that disappeared inside the metal. Set into the concave top was a small round glass-like disk with a knurled bezel around it.

And that was it. No switches, no buttons, no controls of any sort that he could see. For all he could tell, it might have been the inner workings of one of the pull-ring talking dolls his sisters had played with when he was a boy.

He looked up. Jonah was watching him like a dog trainer with a new student. "You don't really want me to just start playing with it, do you?" Fierenzo asked him.

"Why not?" Jonah asked. "You can't hurt it."

"Come on, Jonah, I'm too tired for puzzle box games," Fierenzo said. "Give."

"Okay," Jonah said agreeably. "But if I do, show-and-tell is over. Deal?"

"I don't know," Fierenzo hedged. "There's still those radios of yours, and that invisibility trick—"

"Deal?" Jonah repeated.

Fierenzo sighed. "Deal."

"All right," Jonah said, uncrossing his arms. "Put the flat side against the wall, with the ring hanging downward."

Fierenzo set the device against the wall as instructed about a foot above the rooftop. "Now rotate the bezel around the eye a quarter-turn counterclockwise to loosen it," Jonah instructed.