He knew at once it was a human bone, when he took it from the baby who was sitting on the floor chewing it.

The birthday party had just reached its climax with a deafening noise. The pizza delivery boy came and went and the children gorged themselves on pizza and swigged Coca-Cola, shouting each other down the whole time. Then they jumped up from the table together, as if a signal had been given, and started running around again, some armed with machine guns and pistols, the younger ones clutching cars or plastic dinosaurs. He couldn’t figure out what the game involved. For him it was all one maddening din.

The mother of the birthday boy popped some corn in the microwave. She told the man she would try to calm the children down, switch on the television and play a video. If that failed she would throw them out. This was the third time they had celebrated her son’s eighth birthday and her nerves were stretched to breaking point. The third birthday party in a row! First all the family went out for a meal at an extortionate hamburger joint that played ear-splitting rock music. Then she gave a party for relatives and friends of the family, which was as grand an occasion as if he were being confirmed. Today, the boy had invited his classmates and friends from the neighbourhood.

She opened the microwave, took out the swollen bag of popcorn, put another in its place and thought to herself that she would keep it simple next year. One party and have done with it. As when she was a little girl.

It did not help matters that the young man sitting on the sofa was totally withdrawn. She had tried chatting to him, but she gave up and felt stressful with him in her sitting room. Conversation was out of the question: the noise and commotion that the boys were making left her nonplussed. He had not offered to help. Just sat there staring into space, saying nothing. Desperately shy, she thought to herself.

She had never seen the man before. He was probably aged around 25 and was the brother of one of her son’s friends at the party. Almost 20 years between them. He was thin as a rake and he shook her hand at the door with long fingers, a clammy palm, reticent. He had come to fetch his brother, who refused point blank to leave while the party was still in full swing. They decided that he should step inside for a while. It would soon be over, she said. He explained to her that their parents, who lived in a town house down the road, were abroad and he was looking after his brother; he actually rented a flat in town. He fidgeted uncomfortably in the hallway. His little brother had escaped back into the fray.

Now he was sitting on the sofa watching the birthday boy’s one-year-old sister crawling across the floor in front of one of the children’s bedrooms. She wore a white frilly dress and a ribbon in her hair, and squealed to herself. He silently cursed his little brother. Being in an unfamiliar household made him uncomfortable. He wondered whether to offer his assistance. The mother told him that the boy’s father was working late into the evening. He nodded and tried to smile. Declined the offer of pizza and Coke.

He noticed that the girl was holding some kind of toy which she gnawed at when she sat down, dribbling profusely. Her gums seemed to be irritating her. Probably teething, he thought.

As the baby girl approached him with her toy in her hand he wondered what it could be. She stopped, wriggled herself onto her backside, then sat on the floor with her mouth open, looking at him. A string of saliva dripped onto her chest. She put the toy in her mouth and bit it, then crawled towards him with it clutched in her jaws. When she stretched forward, pulled a face and giggled, the toy fell out of her mouth. With some difficulty she found it again and went right up to him holding it in her hand, then pulled herself up to the arm of the sofa and stood beside him, wobbly but pleased with her achievement.

He took the object from her and examined it. The baby looked at him in confusion, then started screaming for all she was worth. It did not take long for him to realise that he was holding a human bone — a rib, ten centimetres long. It was off white in colour and worn smooth where it had broken so the edges were no longer sharp, and inside the break were brown blotches, like dirt.

He guessed that it was the front of the rib and saw that it was quite old.

When the mother heard the baby crying, she looked into the sitting room and saw her standing at the sofa beside the stranger. She put down the bowl of popcorn, went over to her daughter, picked her up and looked at the man, who seemed oblivious both to her and to the screaming baby.

“What happened?” the mother asked anxiously as she tried to comfort her child. She raised her voice in an effort to shout over the noisy boys.

The man looked up, got slowly to his feet and handed the mother the bone.

“Where did she get this?” he asked.

“What?” she said.

“This bone,” he said. “Where did she get this bone?”

“Bone?” the mother said. When the girl saw the bone again she calmed down and made a grab for it, crosseyed with concentration, more drool dangling from her gaping mouth. The baby snatched the bone and examined it in her hands.

“I think that’s a bone,” the man said.

The baby put it in her mouth and calmed down again.

“The thing she’s gnawing,” he said. “I think it’s a human bone.”

The mother looked at her baby chomping on the bone.

“I’ve never seen it before. What do you mean, a human bone?”

“I think it’s part of a human rib,” he said. “I’m a medical student,” he added by way of explanation, “in my fifth year.”

“Rubbish! Did you bring it with you?”

“Me? No. Do you know where it came from?” he asked.

The mother looked at her baby, then jerked the bone out of its mouth and threw it on the floor. Once again, the baby broke into a wail. The man picked up the bone to examine it more closely.

“Her brother might know…”

He looked at the mother, who looked back awkwardly. She looked at her crying daughter. Then at the bone, and then through the sitting-room window at the half-built houses all around, then back at the bone and the stranger, and finally at her son, who came running in from one of the children’s bedrooms.

“Toti!” she called out. The boy ignored her. She waded into the crowd of children, pulled her son out with considerable difficulty and stood him in front of the medical student.

“Is this yours?” he asked the boy, handing him the bone.

“I found it,” Toti said. He didn’t want to miss any of his birthday party.

“Where?” his mother asked. She put the baby down on the floor and it stared up at her, uncertain whether to begin howling again.

“Outside,” the boy said. “It’s a funny stone. I washed it.” He was panting for breath. A drop of sweat trickled down his cheek.

“Outside where?” his mother asked. “When? What were you doing?”

The boy looked at his mother. He did not know whether he’d done anything wrong, but the look on her face suggested as much, and he wondered what it could be.

“Yesterday, I think,” he said. “In the foundations at the end of the road. What’s up?”

His mother and the stranger looked each other in the eye.

“Could you show me exactly where you found it?” she asked.

“Do I have to? It’s my birthday party,” he said.

“Yes,” his mother said. “Show us.”

She snatched up her baby from the floor and pushed her son out of the room in the direction of the front door. The man followed close behind. The children fell silent when their host was grounded and they watched his mother push Toti out of the house with a stern look on her face, holding his little sister on her arm. They looked at each other, then set off after them.