The crumbly turned away and started climbing up the shelves behind him, throwing aside ancient weapons to clear space for his hands.

Tally turned to Shay, remembering not to use her name aloud. "Why is that guy climbing the walls?

Shay didn't respond, but on the next swing of the rifle, Tally got her answer.

The bottle broke open, and a silvery liquid streamed from it, spreading out across the floor. The liquid flowed into many rivulets, stretching out like some hundred-legged spider after a long nap.

Shay hopped away from the spill, and Tally took a few steps back herself, unable to take her eyes from the mesmerizing sight.

The crumbly looked down and let out a horrible howl. "You let it out? Are you insane?"

The liquid began to sizzle, and the smell of burning plastic filled the museum.

The alarm changed tone, and in one corner of the room a tiny door popped open, disgorging two little hoverdrones. Shay leaped toward them and whacked one with the rifle butt, sending it into the wall.

The second dodged around her and let loose a spray of black foam at the silver liquid.

Shay's next swing choked the spray off. She leaped across the growing silver spider on the floor.

"Get ready to jump."

"Jump where?"

"Down."

Tally looked at the floor again, and saw that the spilled liquid was sinking. The silvery spider was melting its way straight through the ceramic floor.

Even inside the cool of her sneak suit, Tally felt the heat from wild chemical reactions. The smell of burnt plastic and charred ceramic had become choking.

Tally took another step back. "What is that stuff?"

"It's hunger, in nano form. It eats pretty much everything, and makes more of itself."

Tally took another step back. "What stops it?"

"What am I, a historian?" Shay rubbed her feet in a patch of the black foam. "This stuff should help. Whoever runs this place probably has an emergency plan."

Tally looked up at the crumbly, who had reached the top shelf, his eyes wide with fear. She hoped that climbing the walls and panicking wasn't the whole plan.

The floor groaned underneath them, then cracked, and the center of the slivery spider dropped out of sight. Tally gawked for a moment, realizing that the nanos had eaten their way through the floor in less than a minute. Tendrils of silver remained behind, still spreading in all directions, still hungry.

"Down we go," Shay cried. She stepped gingerly to the edge of the hole, peered down, then cannonballed through.

Tally took a step forward.

"Wait!" the crumbly cried. "Don't leave me!"

She looked back - one of the tendrils had reached the shelf he was clinging to, and was swiftly spreading up into the jumble of ancient weapons and equipment.

Tally sighed, leaping up onto the shelf next to him. She whispered in his ear, "I'm saving you. But if you mess with me I'll feed you to that stuff!"

The voice distortion that hid her identity turned the words into a monstrous growl, and the man only whimpered. She prized his fingers from the shelf, balanced his weight across her shoulders, and jumped back down to an untouched part of the museum floor.

Smoke filled the room now, and the crumbly was coughing hard. It was as hot as a sauna, and it was dripping inside Tally's sneak suit, the first time she'd sweated since turning special.

Another section of the museum floor fell through with a crash, leaving a gaping view of the room below. The soccer field full of machines was ribboned with silver tendrils, one of the giant vehicles already half-consumed.

The Armory was fighting back against the hungry nanos in earnest now. Small flying craft filled the air, frantically spraying black foam. Shay hopped from machine to machine, whacking them with the rifle, helping the goo spread.

It was a long drop, but Tally didn't have much choice. The shelves had begun to tilt as the nanos consumed their bases.

She took a deep breath and jumped, the old man on her shoulders screaming the whole way down.

Landing atop one of the machines, she grunted under the crumbly's weight, then dropped to an untouched bit of floor. The hungry silver goo was close, but she managed to dance to a halt, grippy shoes squeaking like panicked mice.

Shay paused in her battle with the sprayer drones for a moment and pointed over Tally's head.

"Watch out!"

Before Tally could even look up, she heard the creaking sound of another collapse. She hopped away, avoiding tendrils of silver and blotches of slippery-looking black foam. It was like some littlies'

game of hopscotch, but with lethal consequences if she made a mistake.

Reaching the other end of the room, Tally heard more of the ceiling collapse behind her. The contents of the museum's shelves rained down on the construction machines, two of which had been turned into boiling masses of silver. The sprayer drones were trying to cover them with black foam.

Tally dumped the crumbly into a heap on the floor and checked the ceiling directly overhead.

They weren't below the museum anymore, but the silver stuff would keep spreading even through the walls. Was it going to eat the whole building?

Maybe that was Shay's plan. The foam seemed to be working, but Shay leaped from safe spot to safe spot laughing, swinging at the sprayer drones, preventing them from getting the outbreak under control.

The alarm changed tone again, shifting to an evacuation warning.

Which seemed like a good idea to Tally.

She turned to the crumbly. "How do we get out of here?"

He coughed into a fist. The smoke was filling even this giant room. "The trains."

"Trains?"

He pointed downward. "Subways. Just below ground level. How did you get in here? Who are you, anyway?"

Tally groaned. Subway trains? Their boards were on the roof, but the only way up was through the hovercraft bay, full of deadly machines that would be very awake by now...

They were trapped.

Suddenly, one of the huge vehicles sprang to life.

It looked like some piece of old farm equipment, the sharp metal threshers across its front slowly beginning to spin. It struggled to turn, working its way out of its cramped parking space.

"Boss!" Tally called. "We need to get out of here!"

Before Shay could answer, the whole building rumbled. One of the construction machines had been turned entirely into silver goo and was starting to sink through the floor.

"Look out below," Tally said softly.

"This way!" Shay cried, her voice barely audible in all the commotion.

Tally turned to pick up the crumbly.

"Don't touch me!" he cried. "They'll save me if you just get away from me!"

She paused, then saw that two little sprayer drones were hovering protectively over his head Tally dashed across the room, hoping the floor wasn't about to collapse. Shay was waiting for her, swinging the rifle to protect a growing web of silver on the wall. "We can get through here. Then past the next wall. We have to reach outside sooner or later, right?"

"Right...," Tally said. "Unless that thing crushes us." The farming machine was still struggling free of its parking space. As they watched, a bulldozer next to it started up, rolling out of its way. The larger machine untangled itself and began to roll toward them.

Shay looked back at the wall. "Almost big enough!"

The hole was widening quickly now, its silver edges glowing with heat. Shay pulled something from one of her sneak suit's pouches and hurled it through.

"Duck!"

"What was that?" Tally shouted, crouching down.

"An old grenade. I just hope it still - "

A flash of light and a deafening roar came through the hole.

"... works. Come on!" Shay ran a few steps toward the lumbering farm machine, skidded to a halt, then turned and faced the hole.

"But it's not big enough..."

Shay ignored her, diving through. Tally swallowed. If one drop of the silver stuff had gotten on Shay...

And she was supposed to follow?

The rumble of the farm machine reminded her that she didn't have much choice. It had detoured around the sinking, infected vehicles, and was in the clear now, gaining speed every second. One of wheels was ribboned with silver goo, but wouldn't be eaten away until long minutes after it had smashed Tally flat.

She took two steps back, put her palms together like a diver going into water, and threw herself through the hole.

On the other side, Tally rolled to a stop and sprang to her feet. The floor shook as the farming machine hit the wall, and the glowing hole behind her was suddenly much bigger.

Through it, she saw the huge machine backing up for another attack.

"Come on," Shay said. "That thing's going to get in here pretty quick."

"But I ..." Tally strained to turn and look at her own back, her shoulders, the bottoms of her feet.

"Relax. No silver ickies on you. Me either." Shay stuck the barrel of the rifle into a drop of silver goo, then grabbed Tally and dragged her across the room. The floor was covered with the charred remains of foam sprayers and security drones that had been destroyed by Shay's grenade.

At the opposite wall, Shay said, "The building can't be much bigger than this." She pushed the half-consumed rifle against the wall. "Hope not, anyway."

A glob of silver clung, already beginning to grow...

The floor shook with a mighty boom again, and Tally spun around to see the front end of the threshing machine pulling back from the hole. The gap was much wider now, big enough to walk through.

Between the hungry goo and the pummeling, the wall wasn't going to last much longer.

The farming machine was now thoroughly infected. Glowing tendrils traveled across its threshers like spinning lightning. She wondered if it would be consumed before it could pound its way through. But a pair of spraying drones shot into view and began to douse it with black foam.

"This place really wants to kill us, huh?" Tally said.

"That's my guess," Shay said. "Of course, you can try surrendering if you want."

"Hmm." The ground shook, and Tally watched as more of the wall crashed to the floor. The hole was almost big enough for the huge machine to roll through. "Got any more grenades?"

"Yeah, but I'm saving them."

"What the hell for?"

"For those."

Tally turned back toward the spreading silver web. The night sky showed through at its center, and Tally saw the running lights of hovercraft outside.

"We're dead," she said softly.

"Not yet." Shay pressed a grenade against the sliver nanos, watched them spread for a moment, then tossed it underarm through the gap, pulling Tally down.

The boom of an explosion battered their ears.

Across the room, the thresher struck for the last time, the entire wall collapsing into glowing silver rubble. The machine rolled forward slowly now, struggling along on half-eaten wheels covered with black foam and shimmering sliver.

Through the hole behind her, Tally saw the shapes of more hovercraft than she could count.

"They'll kill us if we go out there!" Tally said.

"Get down!" Shay barked. "That goo could hit a lifting fan any second."

"Hit a what?"

At that moment, a horrible sound came from outside, like gears grinding wrong on a bicycle.

Shay pulled Tally down again as another explosion rang out. A spray of silver droplets came through the hole.

"Oh," Tally said softly. The nanos on Shay's grenade had been blown onto some unlucky hovercraft's lifting fans, which had let loose a deadly shower as they'd been consumed. By now, every machine waiting for them outside must have been infected.

"Call your hoverboard!"

Tally flicked her crash bracelet. Shay was readying to jump, hopping between the spreading droplets of silver that covered the room. She took three careful steps, then threw herself into the gap.