On the feast day of Saint Justin Martyr, Acha found herself called to the strangers' room - and by no less a person than the abbess herself. Acha tried to steel herself for what she knew would be bad news. It was already summer; the blossoms were fading on the medlar that grew against one wall of the cloister. And though the Abbey itself was far from the sea, even here they prayed for freedom from the raiders who crossed the water when the weather was right. Acha had never seen an attack, but everyone had heard the stories, of raiders in narrow boats who would burn houses and attack the inhabitants as they tried to escape the burning building. Acha had grown up on the shore; her older brother still lived within site of the water. She had always half expected to be called down by the abbess to be told that there had been a raid, and the people she had once loved were dead.

Strictly speaking, the strangers' room wasn't part of the abbey. It was almost a small building unto itself, outside of the heavy limestone walls that separated the congregation from the outside world. Acha wrote her name in the book that recorded when the sisters left or returned to the abbey, then gingerly stepped out into the large, almost empty room.

The dark, wood-panelled room was cool, the hearth nothing more than a dark, empty space. The room still smelled of smoke, of poorly-kept fires lit on damp wood and left smouldering, like the guests whom the abbess could leave waiting for as long as she pleased. This time, the abbess had not left the guest, whoever he might be, waiting long. They made an amusing sight, both of them with their backs to Acha, the squat, middle-aged abbess barely reaching the shoulder of the young man with whom she was conversing. The abbess was dressed in the familiar habit of the sisters here; Acha herself, as a novice, wore a simpler version with a white veil, rather than black. The man wore a stained blue travelling cloak over a red tunic and cream trousers; his hair, falling halfway between his cheekbones and his chin, was the same sandy shade as Acha's own.

"Magistra?" Acha asked, tentatively. It was not often wise to interrupt the abbess. The woman was an effective administrator, to be sure, but she could be sharp-tongued with the novices, and was more interested in what went on in the infirmary than in the artificium where Acha spent most of her time.

The Abbess turned suddenly, a whirl of skirt and scapular, and for a moment Acha was afraid she might be about to face the older woman's wrath. The abbess was completely forgotten, however, the moment the man turned. Acha recognized him.

"Osric!" she exclaimed, throwing herself into her older brother's embrace. He was just as she remembered him; tall, broad-shouldered, and warm - and he was smiling. There was no danger here, no bad news. She leaned into his shoulder, breathing him in, the smell of horse and outdoors and something sweetly musty that was his own scent. Acha only broke the hug when she heard the abbess noisily clear her throat. Cheeks burning, Acha dropped her hands to her sides, and took two steps back. Osric grinned smugly.

"Please conduct yourself decorously, sister," the abbess cautioned. "And do not forget the dignity of your position."

"Yes, magistra," Acha agreed, looking down at the worn wooden floor. She did not look over at her brother, whom she was certain was smiling at her chagrin.

"I shall leave you to converse with your guest, sister," the abbess continued, "And I wish for you to give careful consideration to his requests."

Acha looked up at this, a little confused. She couldn't think of any request Osric might have of her. Unless, perhaps, he wished for her to come visit their childhood home. Acha had no desire to be so close to the sea at this time of year. Or perhaps he was going on pilgrimage, and wanted her to accompany him; she would like to visit Rome or Compostela, she thought.

As soon as the abbess had left the room, Acha threw herself onto her brother again.

"Woah! Enough of that, pet! I'm here on serious business," He said.

"What is it, Osric?" Acha asked excitedly. "Are you on pilgrimage? May I come? I have always wanted to visit Rome, and - "

"Acha!" her brother interrupted. "I am not on a pilgrimage. But if I do go to Rome, some day, you may certainly accompany me. No, my business is more serious than all that."

Acha frowned. She resented the implication that taking a trip that would benefit one's soul was not serious business. Her brother's eyes were good-humoured, cajoling.

"What is it you want of me, Osric?" Acha asked. She could hear the suspicion in her own voice.

Osric's conspiratorial smile broadened. "I hear you've finished the device."

Acha's reply was immediate and firm. "No. You can't have it. It belongs to the abbey. Besides, it isn't quite done."

"Come now, I know it's finished. Your abbess said as much."

"Magistra?" Acha called over her shoulder, but the door back into the main part of the abbey was closed, the abbess ensconced somewhere within.

"Really, pet. She says you've done very well - and after all your letters, I have been dying to see the finished product. I remember all the little toys you used to build when we were children."

"I do recall when you stole my clockwork mouse," Acha admitted. "It took me three weeks to construct the central sprocket alone, and I spent the better part of a day painting the whiskers! And then you made off with it, and I never saw it again."

With a sigh, Osric drew out the leather pouch that hung at his waist. He reached in a hand, and then, sitting on his palm, was the wooden mouse.

Acha squealed with joy and reached for the toy. The curve of the mouse's back still fit her hand perfectly, and when she held it up, the little painted eyes were exactly as they had been when she had last seen the contraption.

"Osric! All this time, and you still had it?"

Osric shrugged. "I found it when I was packing, and I thought I would bring it to you. Perhaps I am trying to make amends. It was nothing but jealously, you know. You were eight and could take a few pieces of wood and make something that moved; I was eleven and could barely paste two pages together to play a prank on my tutors."

Acha felt her cheeks warming with delight. "All I did was that I watched the man who built father's clepsydra - watched and listened. He would have taught you as well, had you been interested."

Acha remembered the clepsydra, a mechanized water clock with a little bell that rang every hour. It was as accurate as a sundial, but worked at night, so long as someone - usually Acha - remembered to refill the water reservoir.

Osric shook his head ruefully. "I never was one for study, Acha. You know that. But I did envy you your little toys. And as I said, the abbess tells me you've finally completed one of the big ones."

Acha frowned. "We have finished something, but I do not see what it has to do with you. The device has no military applications."

"I would appreciate it if you would let me be the judge of that," Osric said. "And besides, who said I wanted it for military purposes?"

"You live by the sea, Osric. The raiders have come twice in the past three years."

"Do you not pray, from the fury of the northmen, deliver us, oh Lord? At least let me see the device."

"I do not care. I will not have you use something I designed to kill people."

"Don't you care about my safety, Acha? About your childhood home?"

"You are a rat, always were, and it hasn't been my home in years."

"Then think of your little nephew -"

"Whom I have never met!" Acha exploded. "I do not wish to help you kill people, Osric. Even if they are raiders who burn churches and kill men and steal children. Don't you understand, Osric? We are to turn the other cheek. To repay violence with violence is wrong. It is a sin, Osric. I intend to take my vows and pursue peace."

"I might mention that those vows include obedience." Osric sounded smug. Acha hated that.

"And so?"

"The abbess told you to take my request seriously. Show me what you've built, little sister."

"I can't. Strange men aren't allowed inside the walls." Even to her own ears, Acha sounded as smug as Osric.

"Acha. I am your brother. I am not strange. And you heard the abbess. I am sure the other women can bear my manly presence for a few minutes. Take me to the artificium."

"I suppose it will do no harm," Acha grumbled. She had already decided she would not allow her brother to take her invention. And he was unobservant - always had been - so what harm could there be in showing him what she had built? As to the other issue, well, Acha could not imagine any of the sisters finding Osric so attractive as to cause them any disquietude.

"Follow," Acha said simply.

Acha led Osric through the door at the back of the strangers' room, pausing to sign her name again in the large book, and to add Osric's, as well. The ink from her previous entry was already darkening towards black, a permanent mark that could only be removed by scraping it off the parchment page. Acha sighed, and led Osric off through the dim stone-walled corridors.

They cut across the cloister, then followed a wide hallway past the scriptorium, and a narrower hallway that separated the infirmary from the buttery. This hallway was busy with sisters carrying poultices and medicines from one room to the other. Acha's cheeks burned at the raised eyebrows and squeals of surprise as they realized Osric was a stranger in their midst. Finally, however, they were out in the sunshine, in the Abbey's huge garden, and, for the moment, they were alone. The young plants were vigorous, medicinal herbs vying for space with broadbeans, leeks, and lettuce. In one corner a patch of asparagus, already past their moment of glory, spread a flurry of delicate fronds towards the sun.

The artificium was at the back of this garden, a former barn repurposed a century ago when the sisters' projects grew too large to be kept in the refectory or the kitchen. It was a stone building with a thatched roof borne on stout wooden beams. Acha could not help smiling when she entered, even with Osric on her heels. Inside, the sunlight filtered in on beams of dust, caressing the cluttered workbenches that lined the walls. The room smelled of wood shavings, but there was not a hint of any visible within the room. It also smelled of smoke, not plain wood smoke, but burning charcoal.

Right in the middle of the room, centred on the stone floor, was Acha's device. It consisted of a huge, wheeled cart, made of metal except for the wooden wheels, with a metal tank sitting on top of it. One each side of the tank were large paddles, which moved pistons housed within the tank. These paddles had been jury-rigged to connect to a series of belts that ran up towards the ceiling.

Osric sighed. "A mobile pump, Acha? The Romans had those before Christ, I'm told. I expected something better."

Acha felt a blush spread hotly over her cheeks. "You are unobservant, Osric. This was a water pump - the abbey's fire engine. But it isn't any more. It is something else."

"Yes, so?" Osric interrupted, one eyebrow raised.

"You know that the Abbey has a large library? There is the book in the abbey library written by one Marcus Vitruvius. De Architectura"

"A book about buildings?" Osric raised an eyebrow. "So what?"

"It is not only about buildings, Osric! It is also about devices. Machines. Pumps and siphons. Even," Acha's eyes were glowing with excitement, " a device developed by an Egyptian Greek called Hero - the aeolipile, which is moved by the boiling of water, in a most clever - "

Osric held up his hand, stopping the flow of words. "Relevance?"

"The relevance," Acha said, the light dying out of her eyes, "Is that I had an idea."

"What sort of an idea?" Osric sounded impatient.

"As I told you, you are unobservant," Acha replied cruelly. "Come over here."

Osric obediently followed his sister around to the far side of the portable tank and pump. On this side, Osric could see that there was a firebox built tight against the side of the tank. Osric could also see that additional piping had been added onto the water tank. He ran a finger casually along one, following it with his eyes.

Acha cleared her throat. "You light a fire there, and it boils the water. This is my aeolipile, so to speak. It is a very small firebox, and a very large water tank, so it takes a while to get going, but it's more powerful that I expected."

"Yes, but powerful at doing what?" Osric sounded exasperated.

"Well, actually, almost anything," Acha said modestly. "See, instead of the paddles moving the pistons and moving the water, the water, by boiling, moves the pistons, which move the paddles. I had to make some changes to the pistons, but it wasn't as difficult as I feared."

"So it's a toy?" Osric asked, dryly.

"No! Not at all. Pump the paddles, Osric?" Acha asked.

Acha scrambled around, grabbing a chain, where it came down from a pulley somewhere high above. She pulled the chain over a wooden sprocket, attached via a belt to a sapling stripped of its bark. The sapling, in turn, was attached by a series of short chains to a half dozen long, narrow dowels.

Osric found he had to lean all his weight into the machine to get the paddles moving. Almost immediately, a grin blossomed onto his face. As he started the pump, the chain rattled to life, and the sapling began to turn, first slowly, and then, as a rhythm developed in Osric's pumping, faster and faster. The dowels spun around, striking the floor forcefully.

"You see?" Acha yelled. The device was not quiet - besides the woosh of the pump and the rattle of the chain, the dowels thudded against the floor rhythmically. "It is a threshing machine," she continued, "If there were wheat on the floor, those dowels would act as flails."

Osric let go of the paddle, stepping away from the machine. It slowed, and then stopped, and the room was quiet again.

"You could connect that chain to anything," Acha said, "and it would work the same way." Her voice was solemn. "In fact, we were able to move the machine by connecting a gear to its wheels."

"I see," Osric said, but it was clear he did not.

"Osric!" He really was the most unobservant man on earth, Acha reflected. "If I were to light a fire, it would be the fire doing the work, you see? Not you."

Osric thought for a long moment. Then he asked, "It really works?"

"Yes! Of course! Ask the abbess if you don't believe me."

"Could it wind a windlass?"

"Of course it could! What sort of stupid question - "

Osric smiled. "I thought you said this had no military applications."

The blood drained out of Acha's face. "What - what do you mean, Osric?"

"If you can wind a windlass, you can hurl stones. Or pull down a wall. Or any number of practical things."

"It is wrong Osric. It doesn't matter who you want to hurl stones at. It is wrong. I was going to use the device to run a loom," Acha said, her lips numb.

"Yes, yes, very sensible." Osric said dismissively. "Can I have it?"

"What? No!"

"But your abbess said -"

"She said to take you seriously, Osric. I did take you seriously - I brought you out here. But I can't give you my aeolipile. I won't." Acha could hear her own heartbeat in her ears. She knew Osric could not force her to give him the device; it belonged to the abbey. And thankfully, through some miracle, her brother had always been unobservant. There was no way he could copy her invention. Her work was safe; Osric could not apply it to doing murder.

"But, Acha - "

"No. Go home, Osric. I will show you out."

Osric dipped his head, running his hand once more along the side of the water tank. "Fine," he said.

Acha's heart was pounding, but Osric quietly followed her back through the garden and the abbey corridors. At the entrance to the strangers' room, Acha once again signed both their names in the book. She then followed her brother through the room, and out onto the street beyond.

"I'm sorry, Osric. It is just that I can't allow you to use my device to kill people. I can't."

"Shh, I know, pet," Osric said with a sigh, "You have your morals, and I suppose that's a good thing."

He leaned over his sister, dutifully kissing her on the forehead.

"Farewell, Acha. I really will take you to Rome, if I ever go."

"I know. Farewell."

And Osric turned, and was gone, and it was done.

As the bells started ringing for Sext, Acha realised she was still holding her toy mouse. She turned the mouse over, reveling in the old, familiar feel of the toy in her hand. Suddenly, something struck her as odd. She had grown in the years since she had last seen the mouse; her hand was bigger than it had been when she was eight. Yet the mouse still fit her hand perfectly.

With a shock, Acha realised the truth. This wasn't her mouse, but an almost-perfect copy. The only living person who had ever seen the original, besides Acha herself, was Osric. Osric, who though unobservant, had observed well enough to copy the toy years later. Osric, for whom Acha had just proudly demonstrated the aeolipile, with all its possibilities.

Acha thought of Osric running his hands thoughtfully over various parts of the machine, and she felt sick.

But it was too late; the damage had been done, and besides, the bells were ringing . She needed to get inside, to the chapel, to join the sisters in prayer. With the back of her hand, Acha dashed the tears from her eyes The bells pealing in her ears, she walked back through the door.