By Turns

The alarm chirped every nine seconds, an interval Ilsa had chosen herself and was prepared to call a mistake.

Her arm was in the plenum to the elbow, water at thirty-one degrees, salt and warm iron. The metering nozzle was a sintered thimble behind the diffusion segment, half-blind, and the line had been starving for two hours. She worked the bent wire into its throat. Nothing. She turned it the other way, which the manual forbade, and a grey curl of deposit came out on the hook.

"Ha," she said, to nobody, refitted the nozzle, and cracked the isolation valve. The line took the water with a sound like a swallowed cough. The chirp stopped.

That was the work order: metering nozzle, feed train two, cleared and tested. Three days to her shuttle, a handover sheet one line short of no open items, and then Petra's workshop, where a bench had her name on it. It would not close. The order carried a verification clause — no unresolved sealer events in the shift window — so she pulled the Mendery's log onto the tablet on her chest.

The gantry rode rails along the plenum wall and sealed any apparent tear in the black diffusion sheet within seconds, because a tear concentrates the feed into one jet and starves the rest of the sheet. Good engineering.

The doses varied by two percent. The spacing held to the millimetre. The timestamps sat at 02:10 nearly every night for eleven weeks, three holes then one, every entry filed as membrane abrasion, maintenance. She had countersigned all of them, initials and date, and none had reached Adaeze's behavior dataset, because the software had decided what it was. The first animal brought home from another star, and most of its records were in her maintenance queue.

A week ago Adaeze had asked for a probe port in the segment frame. Ilsa had said no — a biologist's hands in a plenum, another delay in a full maintenance schedule. She had not been wrong about the schedule. She dried her arm and took the tablet to Adaeze.

Adaeze's store room had a bench, a kettle, and a screen of numbers moving for a year. She read the log twice.

"The gantry steps in increments," she said. "Three and one could be three steps and a return. The spacing is its own."

"The doses match within two percent."

"That's a sealant valve. It isn't a signature." Adaeze scrolled. "Or it's stereotypy. A confined animal works one soft surface the way a caged one paces. A husbandry note, not a conversation."

"I didn't say conversation."

"You brought it to me at midnight." Adaeze set the tablet down. "I spent a season on a rhythm once. Ten weeks, before I found out it was the feed pump's own cycle. I'm not finding a mind in a gantry log for you."

"Then find me something that isn't a rhythm."

Adaeze was quiet. "If it's cutting for flow, the punctures follow the feed, not the clock. The pump is the clock. The feed is the variable." She looked up. "The feed is yours to switch."

The plenum was warm as a bath, a hand's width across. Ilsa lay on her side with her cheek to the frame, set the feed to half, and put two fingers on the back of the black sheet.

Three slow pushes, a second apart. A pause. Then one longer, harder push that moved her fingers. She checked her watch: 02:08.

She shut the isolation valve. Four minutes by the watch, through 02:10, water still around her arm, the gantry idle with its laser fan dark. No puncture. No sealer event.

She opened it. The gantry crossed before she felt anything: a hole where the last one had been, low on the sheet, sealant down, flow evening out. Then nothing. No second hole. No third. Ilsa kept her fingers on the sheet and understood, after a while, that it was waiting.

Not proof. A live hypothesis, and it had eaten her clean handover. Her arm had gone numb. She was staying.

She called the duty desk, gave back her sleep slot, and queued the controller in the fabricator: twelve hours of print, a driver to hold the sealer off, the tank on manual feed for a day. Three spares left. Then she pulled the eleven weeks of 02:10 entries from the maintenance queue and flagged the batch for review with Adaeze.

The handover sheet stayed on the board by the hatch, its last line still blank.

Back in the plenum she cut a length of jumper wire, stripped both ends, and laid it across her knee. Then she put her fingers on the black sheet again. Three slow pushes. A pause. One longer. She left her hand there until her wrist ached.

The plenum at 03:20 smelled of warm salt. Ilsa lay on her side with the jumper across her knee.

"Hand valve first. Left hand. It stiffens at the quarter turn, so go slow and don't fight it."

Adaeze turned it too far. The line pressure dropped; the sealer's trigger went amber.

"Prime it first," Ilsa said. "Else the drop trips the trigger and you've sealed nothing and learned less."

"I did prime it."

"Upstream. Downstream holds a cup and a half." Ilsa reached past her and bled it. The light went green.

Adaeze did it again, both sides this time. Her hands were slow. They were exact. Ilsa stopped talking.

"Jumper," she said. "Both contacts. The sealer keeps counting, it just doesn't fire."

Adaeze clipped it, checked it, checked it again. "What's the sealer actually protecting?"

"The flow."

"Where?"

"You know where."

"Say it, so we mean the same thing."

Ilsa let out a breath. "The segment seeps evenly. A tear isn't a breach, it's a concentration. One jet takes everything, the rest of the sheet starves, and the animal gets fed in one place and nowhere else. The sealer closes it in seconds, because seconds is how long it takes to become a husbandry failure."

Adaeze nodded slowly. "It's protecting distribution. Not the barrier."

"Nobody's touching the barrier."

"Then for eleven weeks it's been protecting the distribution from the animal." She took the valve again, primed it, and held pressure with her thumb without being told.

Ilsa looked at the black sheet. "Yes."

The trial was a deck of cards and two controls. Ilsa shuffled; Adaeze turned.

SHUT. Ninety seconds, no puncture, no sealer event, the log flat. Four windows, four times nothing.

OPEN. Within forty seconds: three holes close together and one apart, the same group as eleven weeks of 02:10.

"The gantry steps in fixed increments," Adaeze said.

"The gantry doesn't know the card."

"No. And a fixed rhythm would keep going with the feed shut."

"Unless it sleeps through four windows," Ilsa said.

Adaeze turned REPLY. Ilsa cracked the hand valve, metered one pulse, closed it. The sheet held still. Then a hole, low, where the last one had been. One push of water.

NO REPLY. Two punctures, spaced. Then nothing until the flow changed.

At the end, cards face down, lamp low, neither of them talking, it punctured low on the segment where the sag made the jet strongest, a hand's width from the probe port Adaeze had asked for a week ago. Then it held still.

They waited. Nothing. Ilsa's wrist ached from the valve.

"Feed it," Adaeze said.

Ilsa restored the flow. No puncture. A minute passed.

"One pulse," Adaeze said.

Ilsa sent one. The answer came once, at the new spot, and stopped.

Adaeze sat back on her heels. "It wanted flow there. And it waited instead of repeating." She was quiet a moment. "That's all I'll put in the log."

"Put it in those words," Ilsa said, "and it'll hold."

The controller came out of the fabricator that afternoon, warm and smelling of cured resin: printed housing, hand-soldered jumper, the sealer's trigger routed through a keyed switch. They had it in by 15:00.

On the morning of her shuttle, Ilsa stood at the plenum hatch with her hands in her pockets.

"Run it."

Adaeze ran the last trial alone. Card deck, windows, valve. She opened the feed gate on her own count. Ilsa's thumb twitched toward the bleed; she left it in her pocket. Adaeze closed the gate early — Ilsa would have held it longer — and the trial held anyway, because it didn't depend on Ilsa's count. It never had.

Then the handover sheet.

Ilsa wrote the eleven weeks of misfiled punctures out in her own hand, corrected the reason codes line by line, and signed the batch at the bottom. Then, on the line that had always read No open items, she wrote the first open item of her career: Feed-dependent puncture response. Procedure and controller with Dr. Nwosu. Three spare segments. Bypass keyed to new driver. Ref. this sheet.

Adaeze read the entry and countersigned beside it.

She clipped her pen to the board. It had hung there four years. She left it.

"Shuttle's in an hour," Adaeze said.

"I know."

Adaeze didn't try to change her mind. She turned back to the valve and the cards.

Ilsa went out through the hatch. The corridor was cold after the plenum. Behind her the hand valve ticked once, and the answer came through the wall — one hole's worth of flow, the pipe's note dropping and rising. Adaeze's voice followed on the speaker, flat and careful. "It answered again."

"Log it," Ilsa said, and went to catch her shuttle.