Mycroft lay awake in his bed, watching the shadows from the tree branches outside flicker and quiver across the ceiling. Quick flashes of anaemic moonlight rent through the gusts of cloud-cover, beaming like the search beacon of a distant lighthouse.

He knew he'd been asleep for hours, because the bedside clock said it was nearly ten past two. But the house was neither dark nor silent. There was a chink of light shining under the closed door, and the sound of Mummy's hesitant footsteps in the hall. And a more distant sound - an incessant, pitiful wail that had become the usual nightly background noise since, three weeks before, Mummy had presented Mycroft with a brand new baby brother.

At first Sherlock was just a screaming thing down the hall, or a mysteriously wrapped white blanket being shunted from room to room. If it weren't for all the fuss and noise he made, Mycroft might have wondered if it wasn't some elaborate hoax and he didn't really have a little brother after all.

Dad was in Milan on business again, and Mummy had told her disappointed firstborn that he mightn't be back before the end of the holidays came and it was time to go back to school. And this summer, Mycroft had seen little of Mummy either - thanks to the screaming white blanket, he imagined.

She was in her bedroom, mostly, though Mycroft couldn't imagine what she could be doing in there. Every now and again she emerged and went across to the nursery, clumsy and dazed. Always barefoot and in her nightgown, even if it was the middle of the day. There would then be half an hour or so of blissful silence from behind the nursery door; inevitably, she would return to her room and that thin fretting noise would start up again.

The noise was more than a fretting this time, and it wasn't stopping.

Mycroft stole across the room in his bare feet and edged the door open. A dull halo of light from the nursery at the end of the corridor, but there was no sign of Mummy at all. He tiptoed down to the nursery door and across to the cradle at the far end.

Just fed.

Not soiled.

Not sick.

And waving his tiny arms and legs in the air hysterically by now. Mycroft would never have supposed something so small, so fragile-looking, could make such a robust howling noise. Why couldn't Mummy hear it? Mycroft had a moment wondering if Dad could hear it, and he was in Italy.

And it wasn't just Sherlock's voice that was getting a good workout; his chubby little legs had managed to kick aside his blanket. Yellow and white - the same yellow and white shades of the cotton singlet was wearing.

Sherlock didn't own anything blue. Mycroft had been promised a sister.

All the same, Mycroft imagined a sister would probably scream just as loudly as a brother, so there didn't seem to be a lot of difference there. He slipped his hand between the painted wooden rails of the cradle and rested it on Sherlock's chest. Patted him gently, hesitantly. He was so little. Maybe he was easily broken.

"There now, be quiet," he muttered. So far, he hadn't heard anyone talking to Sherlock, and didn't know what you were meant to say to a baby. "Be quiet…"

Quick as a cobra's bite, Sherlock grasped Mycroft's thumb. He squeezed it in one tiny hand, lifted it, seemed to be inspecting it. Mycroft instinctively went to pull his hand away. Sherlock held fast; then he crammed Mycroft's thumb into his mouth and gummed it for a few seconds.

Not food, apparently. But by now the hysteria was gone; he was hiccuping, but that didn't seem to be bothering him.

Mycroft offered Sherlock all of his fingers in turn until they were gluey with thick baby-drool. He wiggled them. Gently touched Sherlock's face, his ears, his own hands.

Sherlock had apparently decided that his brother's fingers were the most interesting thing he'd ever seen.

Mycroft had no idea at all how old a baby needed to be before they learned how to reason, how to talk, how to count. But it surely couldn't hurt to give him a head start in learning. In little more than a whisper, he counted off his fingers. One, two, three, four, five. Then the names. Pollex, Digitus Secundus Manus, Digitus Medius, Digitus Annularis, Digitus Minimus Manus.

He wiped the drool - it was really too disgusting - on Sherlock's blanket. Undeterred, Sherlock latched back onto his finger - Digitus Secundus Manus - with that surprisingly powerful grip; but by this time he was making a valiant effort to keep his eyes open.

When Mycroft was quite sure his brother was asleep he freed his finger and crept back out of the nursery, almost frightened to breathe lest Sherlock wake and start shrieking with rage about it. As he crossed the top of the stairs he looked down into the pools of darkness below, where something had caught his eye.

Mummy was sitting on the bottom step. Mycroft could have been mistaken, but it seemed to him that she was curled up with her head on her knees, and her hands grasping the back of her neck.