"Dad, what does 'polygamy' mean?"

Sherlock pronounced it "polly-gamey"- a sure-fire sign he'd never heard the expression in real life and had read it somewhere. The Encyclopedia Britannica, perhaps, or he'd been raiding Mycroft's book collection again. This was something of an achievement in itself, since Sherlock was four years old and not yet at school.

"Sherlock, your mother and I are talking," was the curt response.

Family meals had almost become extinct in the Holmes family. But Dad had a six-week stay at home that spring, in between assignments to Cairo and Geneva, and so Mummy had gone to the effort to have things nice for him.

And that was fair, too, Sherlock thought. His entire concept of Egypt had come from the pictures in Mycroft's books, and the little he'd been able to coax out of him to explain those pictures. So if Dad had recently been eating camel-meat on a sandy blanket - next to the Sphinx, of course - with a bunch of Bedouin travellers... maybe that's why the silverware had to be so shiny.

Neither Mycroft nor Sherlock were used to wearing suits to dinner, and Sherlock was not used to wearing a suit at all. He hated the scratchy shirt, and he felt like the tie was choking him. He'd been pulling at it at intervals for the past hour, alternatively contrite and defiant of the scolding he got from Mummy for doing it. Mycroft, she explained, wasn't pulling off his tie, was he?

Mycroft was dressed like Dad. Sherlock didn't know if he liked that either.

As for Mummy, she was in her best apricot-coloured dress - a dress everyone teased Sherlock with by referring to it as the shuttlecock dress, after one mortifying incident where he'd mixed up the two long-tailed words. Sherlock loved Mummy's shuttlecock dress. Philippa Devereaux Holmes was never to know how her youngest child idolised her when she wore it. A princess. A blonde woman in a pink dress just had to be a princess.

Aside from the wonder that was the Shuttlecock Dress, Mummy had all her best jewelry on and her hair pinned up. She'd laid out the best china, most of which Sherlock was forbidden to even breathe near, let alone use - she'd accepted the visual interruption of Sherlock's child's dishes at the table over the possibility of the Wedgwood being broken. Sherlock, for his part, had wolfed down his meal in about two bites. He'd forgotten Mummy's rule: no leaving the table until the grown-ups did.

"But I'm bored," he said in a little voice. "Can't you just tell me quickly, and then talk to Mummy about the Stuck Market again?"

Dad sighed heavily, and Mycroft, after shooting him a nervous glance, cleared his throat. "What did we say about etiquette yesterday, Sherlock?"

"Etiquette is spelled E-T-I-Q-U-E-T-T-E," Sherlock parrotted obediently. "It means being polite."

"And do you think interrupting a grown-ups' conversation with a question like that is being polite?"

Sherlock pouted. He knew that the answer was "no", but he couldn't see why the answer was no. And it really wasn't solving his problems, either; on top of being bored, and not knowing exactly what "polygamy" meant, he was also feeling more energetic than usual. This was only drawn to his attention when he looked across the table and saw Mycroft glaring at him again, fork suspended between his plate and his mouth.

Stop it, he mouthed.

Mycroft saw everything.

Sherlock stopped for approximately ten seconds. Then he forgot himself and there was more wiggling, and he started muttering to himself in a sing-songy voice: "polly-gamey, polly-gamey…"

"It's pol-IG-amy," Dad said, putting his own fork down with an exasperated sigh. If there was anything that Sir Antony Linwood Holmes hated, it was flagrant abuse of the English language, even if the culprit happened to be little more than a toddler. "And it means when a man has more than one wife at the same time." The Holmes children were never patronised when they asked what something meant.

"Oh," Sherlock said, still rocking on his chair and ignoring Mycroft's death-glares. "Like how you've got Mummy and Jemima?"

Silence. Mummy put her glass of wine down and cleared her throat.

"Antony, what did he just say?" she asked in a low voice. Her husband ignored her; she turned her attention to her small son. "Sherlock, what did you say, darling? Tell Mummy again."

Sherlock was too young to hear the controlled anger in his mother's voice, and supposed she was just trying to have a conversation with him about this interesting new subject. "Well, wifes sleep in the same beds as husbands," he explained, "and Dad's your husband. But then on Wednesday when you weren't home, he went to bed in the daytime for a nap with Jemima. Is Jemima Dad's wife, too?"

Jemima Forrester was Sherlock's babysitter.

"Sherlock." Dad sounded much sterner than Mummy. "You've been told not to tell lies, haven't you?"

"I'm not lying," Sherlock protested, confused. "I saw you, it was on Wednesday..."

"Antony," Mummy said, confusing Sherlock even further. She was smiling, but there was something... odd... about the way she spoke. "Would you care to explain what the hell just came out of our son's mouth?"

"Calm down, Pippa, he -"

"I'm not going to calm down; tell me what he's talking about!" She threw her fork onto her plate, where it landed with a sharp crack. Sherlock shrank back in his seat, frightened.

"You know he -"

"He's got too much to say for himself, God knows that, but he doesn't lie. He doesn't lie, Antony. I don't think he knows how. So what the hell have you and Jemima -"

"Sherlock." Dad stood up and threw his napkin down, cutting his wife off mid-sentence. "You know I'm not going to tolerate lies. I want you to come into the study with me."

Sherlock had never been summoned to "Dad's study", and had no idea what was meant by the phrase. But Mycroft had been there on a couple of occasions, and had borne the welts from his father's belt to prove it. He also got to his feet. Dad turned to him.

"Did you have something to say, Mycroft?"

"Yes," Mycroft said in very deliberate tones. "I don't think that you want to hit him."

"Don't I?"

"No. For God's sake, he's four years old, and not to blame for your shortcomings as a husband."

Mycroft was heavy-set and nearly at full height, though he was still a few months off fifteen; easily a match for his father, who knew it. He'd also known for quite some time about Dad's daytime naps with Jemima.

"How dare you speak to me like-"

"Now, I am going to take Sherlock upstairs to bed, and you can discuss the finer cultural points of polygamy with your wife." Mycroft went around the table, pulled Sherlock's chair out, and picked him up. It was the first time he'd done so since Sherlock had learned to walk. "Goodnight."

The shouting match had started before Mycroft reached the top of the stairs.

"Why is Mummy so angry?" Sherlock asked as he put his favourite green "car pyjamas" on. He'd never heard his parents argue before. They'd seldom inhabited the same physical space for long enough to argue.

"Because once again you've ruined everything," Mycroft snapped. "Why can't you ever just shut up, Sherlock? You're always talking and you never think-"

Mycroft stopped, as abruptly as he'd started. Sherlock was looking at him in open-mouthed shock. Shock and, his brother perceived, deep hurt. Mycroft, the Mycroft he was convinced made the sun shine and the rain fall, had never had such harsh tones for him before.

Mycroft felt a hot wave of anger - at himself. But when Sherlock's big grey eyes filled with tears, he shook himself out of it. "No, don't you dare cry," he said. "You're far too old to cry. Just get into bed and go to sleep."

"Can I have a story?" Sherlock asked him in a little voice.

"You can read your own story, Sherlock."

Downstairs, something smashed.

Sherlock was well capable of reading his own book, but it had never happened this way before. Mycroft sat down in the chair by the door while Sherlock was reading. He didn't turn the light out or wish him goodnight or leave the room. He just sat there.

Downstairs, Mummy and Dad were both shouting.

"All right, you've finished that page, that's enough reading." Mycroft confiscated the book straight out of Sherlock's hands and put it back on its shelf. "And no, we're having none of your just-one-more-page routine either. Ready?"

Sherlock knew better than to challenge Mycroft's on the just-one-more-page routine. He settled into his pillows and nodded. Mycroft turned out the light, but there was enough light from the hall for Sherlock to see that he'd sat down in the chair again.

"Mycroft?"

"Go to sleep, Sherlock."

"Why is Dad angry with me?"

Mycroft's grimaced. Sherlock had just heard, as well as he had, Dad describe him as a lying little brat. If you're so determined to take the lying little brat's word over mine then there's not much more to be done here, is there?-

"I said go to sleep, Sherlock. Now."