Porcius corrected her patiently. "The horrea," he said. "The granary."

"But the words are related," said Flavia. "Horrere means to bristle. So horrea means a place where bristly grain is stored and horrere is when your hair bristles because you're scared!"

Nubia nodded. Flavia was one of the cleverest people she knew. Her former mistress loved words and stories, puzzles and codes.

"Whatever." Porcius turned back to Nubia. "After we choose the best grain we grind it to flour. Come into the next room."

"Oh!" Nubia gasped as they moved through the doorway, and her hand went to her throat. "The poor, wretched creatures!"

They had stepped into a spacious room with two big millstones and one smaller one, all made of grey stone. Each was shaped like the hourglass Alma used in the kitchen and around each one paced a blindfolded donkey.

"Why are you blinding them?" Nubia looked at Porcius.

"The blindfold doesn't hurt them," he said. "It's to stop them getting dizzy. They go round and round all day. Look! You can see they're yoked to a beam." He led Nubia to the smaller millstone. The others followed.

A thin slave with hair the colour of dirty straw was pouring grain from a bag into the top of one of the millstones. He nodded respectfully at Porcius.

Porcius ignored him and pointed to the millstone. "It's made of special Etruscan rock. The top stone is called the catillus and it fits over the bottom stone: the meta. You can't see the meta, but it's shaped like a cone: just like the turning-point at the races. There's a gap between them just wide enough to let the grain in. As the donkey pulls the top stone around the grain is crushed between the two stones. Then it comes out as flour." Porcius pointed to a trough.

But Nubia was not looking at the trough. She was looking at the place where years of donkeys' hooves had worn a ring into the stone floor.

"See the flour coming out?" Porcius was saying to her. "This millstone makes the finest flour for our best rolls."

"He seems wretched," whispered Nubia. "Behold where his fur is rubbing off on his shoulders."

"It doesn't hurt them much." Porcius said. "Animals don't feel pain like people do. Come on, Nubia. I'll show you where we mix the flour with water to make the dough."

Lupus followed the others to a room with several large troughs made of the same grey rock as the millstones. A big man stood facing them at one of the troughs. He was turning a vertical wooden bar. As they approached, Lupus could see that the bar rotated blades which mixed the flour and water into an elastic dough.

The slave wore a one-sleeved tunic. As they approached, he looked up at them, and Lupus saw the brand on his forehead: TENEME. Lupus knew it meant "hold me" and that only slaves who had run away were branded on the foreheads like this. The big slave gave Lupus a wink. His muscular chest and arms gleamed with sweat.

Lupus nodded back and then looked down into the trough.

The mass of dough was round and smooth and slightly greyish. "We bake the best bread first," Porcius was saying. "Then in the late morning or afternoon we bake the panis popularis." "What's that?" asked Nubia.

"You know that the Emperor distributes free grain every day? Most people make porridge out of it but some bring it to us to make into bread. It's cheaper because we only charge for the cost of milling and baking. That's panis popularis."

"The Emperor is most benevolent," said Nubia, "to give free bread."

"It's not the bread that's free," corrected Porcius. "Just the grain. And the people would revolt if they didn't get it."

"We don't get free bread," said Jonathan. "I mean, free grain."

Porcius looked Jonathan up and down. "Are you a Roman citizen?"

"Yes," said Jonathan. "But only recently . . ."

"Then you should get a token soon. For some of our special customers we pick up the grain and bake the bread. You have to give us your token. I'll ask my father to put you on our list. Hey!" Porcius looked around. "Where's Nubia gone?"

Flavia spotted Nubia first: she was standing near the mill with the smallest donkey. The slave with straw-coloured hair stood beside her. He had unharnessed the donkey and Nubia was stroking its soft grey head.

"He is so little." She looked up at them as they came in. "Not even as big as Ferox."

Flavia nodded. Ferox was her uncle's guard dog - a huge mastiff.

"You like animals?" said Porcius. "Would you like to see my pets?"

Nubia's golden eyes lit up and she nodded.

"Come on, then." Porcius led them back through the store room and up a flight of dark stairs.

At the top of the stairs they met two women coming down. The first wore a light blue woollen stola with a dark blue palla draped over her head. Her face was dusted with white powder to make her look very fair. Behind her came a younger woman with frizzy brown hair and bad skin.

"Oh, hello, mater," said Porcius. "Are you going out?"

"Yes, dear," said the woman in blue. "Aren't you going to introduce me?"

"This is my mother: Fausta," said Porcius. "Mother, this is Nubia. And some friends of hers. I've been giving them a tour of the bakery." He didn't bother to introduce the woman behind his mother so Flavia deduced that she must be a slave-girl.

"How nice," said Fausta, patting his arm vaguely. She and her slave-girl continued past them. Flavia lingered at the top of the stairs, and noticed that the slave girl carried a bath-set: a bronze ring with strigil, tweezers, ear-scoop and oil-pot attached. Porcius' mother and her slave-girl were certainly going to the baths.

Flavia watched them out of sight, then turned and ran to catch up with the others.

"This is my room," said Porcius, leading them into a bright room with a small balcony overlooking the road. "And these are my steeds."

Nubia uttered a cry of delight. On the table beside the bed was a delicate wooden cage. Nestled in the sawdust were several mice. Nubia counted at least five of them.

"And this," Porcius gestured towards a big wooden box as long as his bed, "is where they race."

The box was open at the top and as Nubia looked down she saw a wooden model of a racetrack. She recognized the layout because she had been to the races in Rome a few months earlier.

"I call it the Circus Minimus," said Porcius, "because it's the smallest racecourse in the world. Pater helped me build it."

"You race your mice?" Jonathan raised an eyebrow. "Yes," said Porcius, setting the cage in the middle of the racetrack and opening the door.

One by one the mice emerged from the cage. Some were grey and some were white. With bright eyes and twitching noses, they explored the model racetrack. There was a central island and the wood round the edges was carved in steps to represent the spectator seating.

The four friends knelt beside the track and Nubia picked up one of the white mice. She giggled as his warm little body explored the palm of her hand.

"What are you doing?" came a voice from the doorway. "Racing those silly mice again? Why don't you grow up?"

Nubia and the others looked up. A girl of about fifteen stood there. She wore a sage-green tunic and had tied a lavender palla round her hips in a way Nubia knew was fashionable among young women in Rome. With her pale skin and dark wavy hair she would have been very pretty except for one flaw: she was cross-eyed.

"Who are you?" she asked, staring at Nubia and the others.

Flavia jumped to her feet. "I'm Flavia Gemina, and these are my friends: Jonathan, Nubia and Lupus. You must be Titia."

"I know you," said Titia. "You're Aristo's pupils." "That's right! Do you know him?"

Titia stood in the doorway for a moment without replying. Nubia couldn't tell which of them she was looking at. Abruptly Titia moved away from the doorway and Nubia heard her footsteps disappearing down the hallway.

"How rude!" muttered Flavia, sitting down again.

"Don't mind her," said Porcius. He had brought out two tiny wooden chariots, each with a wooden rider fixed inside. "She's always in a bad mood because she'll never find a husband."

"Why not?" asked Nubia.

Porcius snorted. "Didn't you see her? She's a cross-eyed freak. I think she's in love with your tutor Aristo," he added.

"How did you know that Aristo is our tutor?" asked Flavia.

"Everybody in Ostia knows. Or at least everybody at my school. They're all jealous because you have a private tutor who is nice to you and lets you do projects and doesn't beat you when you get a sum wrong."

"What school do you go to?" asked Jonathan.

"The one in the forum," said Porcius. "But I don't want to talk about school. Here, Nubia," he said, handing her one of the model chariots. "Why don't you harness up- the Greens and I'll do the Blues. The Greens are the white ones," he added.

"He is lovely," said Nubia, stroking her white mouse.

"That's Cupido," said Porcius. He had already harnessed a grey mouse to his blue chariot. "You can tell him by the nick on his tail. He's best on the inside, because he's steady. Icarus there - the one climbing into the Emperor's Box - he's the fastest so you should put him on the outside. Castor and Pollux are the other two. They go in between."

"You really like the races, don't you?" said Jonathan, handing Icarus to Nubia. She nodded her thanks and smiled as the mouse nosed into his tiny harness. The little creatures were obviously used to the procedure.

"I love the races," said Porcius, without looking up. "I'm going to go to chariot racing school in Rome next year, when I'm twelve."

"They have a school for that?" asked Jonathan.

Porcius nodded.

"Have you ever been to the races?" asked Flavia.

"Myriads of times," said Porcius. "Pater always takes me up every year for the Ludi Romani."

"We've only been once," said Flavia. "But we loved it." "They've been, but I haven't," said Jonathan glumly. "OK," said Porcius, breathing heavily as he concentrated on getting his last mouse into the harness. "Before we put them on the starting line we have to place our bets. I wager ten pistachio nuts on the Blues."

"Five on the Greens," cried Flavia.

"Me too five greens," said Nubia.

Lupus held up his wax tablet: FIVE ON THE BLUES.

"Lupus! You traitor!" cried Flavia. "Why don't you bet on Nubia?"

Lupus grinned and shrugged.

"He obviously thinks he knows who's going to win," said Jonathan.

"Aren't you going to bet?" Porcius asked Jonathan.

"My father doesn't allow it."

Porcius shrugged. "You can be the Emperor, then. You drop the napkin."

"What?"

"To start the race."

"Oh. All right." Jonathan pulled his handkerchief from his belt and held it up.

"How many circles?" asked Nubia.

"Seven circuits, of course," said Porcius. "Just like the real races. Ready Jonathan? Then give the signal . . ."

Flavia wiped tears of laughter from her eyes. "That was the funniest thing I've ever seen. They're so cute. And so fast!"

"Yes," said Porcius proudly. "They're my boys. Here you go, Nubia and Flavia. Here are your winnings. Well done. Come on Lupus. Pay up."

Lupus scowled. He didn't like losing.

"Well run, boys," Porcius said to the mice as he scattered a handful of sunflower seeds onto the Circus Minimus. The mice had been released from their tiny harnesses and they happily devoured their reward.

"Oh!" said Flavia suddenly. "We're supposed to be solving - I mean studying - how bread is made."

"Do you really want to go down to the boring old bakery again?" Porcius asked Nubia. She was holding Castor and stroking his tiny shoulder blades with her finger.

"Can you show us the ovens?" Flavia asked.

Porcius ignored Flavia. "Nubia?" he said. "Do you want to go back down?"

"You can see the little donkey again," said Flavia. Nubia's eyes lit up and she nodded firmly.

"Somebody whipped him!" cried Flavia.

The thin slave with straw-coloured hair was removing bread from a wall-oven. His one-armed tunic left a shoulder exposed and she could see the red welts on his back.

Porcius nodded. "Pater beat the slaves yesterday to see if they knew anything about some missing bread rolls." He noticed the look on Nubia's face. "It doesn't hurt them much," Porcius said. "Slaves don't feel pain like other people."

"Can we try some fresh rolls?" asked Jonathan. "They smell delicious. And I love warm bread."

"No," sighed Porcius. "My father keeps strict account of every loaf of bread baked and sold. If even one goes missing he knows about it. He doesn't even let my mother take bread without asking. Oh, hello, pater!" said Porcius. "These are some friends of mine. They're studying how bread is made."

A short man in a flour-dusted apron came through the doorway from the mill room. A tall boy of about thirteen trailed behind him.

"I know you," said the short man to Flavia. He had onion breath. "You're Alma's mistress."

"That's right. I'm Flavia. These are my friends Nubia, Jonathan and Lupus."

"Titus Nasenius Pistor. This is my eldest son Quintus. We call him Ericius."