“If I had one wish in life,” said Mei Yuan, “it would be that I could spend the rest of it doing nothing else all day but reading.”

Li remembered the books lining the wall in Chao Heng’s apartment, and wondered if he had read them all.

Margaret bit into her jian bing and crunched it cautiously. “Hmmm,” she said enthusiastically. “It’s fantastic.” And then quickly, “It’s not going to burn the mouth off me, is it?”

Li laughed. “Not this time. Just a gentle chili burn.” And she felt the slow warmth and the flavours fill her mouth.

Mei Yuan handed one to Li. “So,” she said. “You have something for me today?”

Li’s mouth was full of jian bing. He shrugged apologetically. “I haven’t had a chance to think of anything, Mei Yuan. I have a big investigation on.”

She waggled a finger at him. “This is no excuse.”

“Okay,” he said, his mind turning over quickly. “What about this? A man commits three perfect murders on the same night. There is nothing to connect them to one another, or to him. But he quite deliberately leaves a clue beside each victim which makes it clear that they were all killed by the same man. Why does he do this?”

“That’s not fair,” Margaret said.

Mei Yuan looked at her, puzzled. “You know the answer to this?”

“No, I don’t.”

“Then it must be very difficult.” She thought for a moment. “What sort of clue does he leave?”

“A cigarette end. He is clever enough to know that traces of his saliva will remain on the paper and that the police will discover that the DNA in the saliva from each cigarette end is the same.”

She looked from one to the other. “Is this real?”

Li nodded grimly. “I’m afraid it is.”

“Then I will think about it,” said Mei Yuan seriously. “And if you ask me tomorrow I will tell you what I have thought.”

Li smiled. “I only hope I will know the answer by then, so that I can tell you if you are right or wrong.”

In the Jeep, as they headed north through the traffic chaos of bicycles and buses in the narrow Chaoyangmen Nanxiaojie Street, Margaret said, “Why is a woman like that selling fast food on a street corner?”

Li shrugged. “The Cultural Revolution ruined many lives in China. Hers was only one.”

Margaret shook her head in exasperation. “What exactly was the Cultural Revolution?” And she was immediately embarrassed by her ignorance. “I mean, I know I should probably know. But it was a long time ago in a far-off place . . . from America, that is.” She flicked a glance at him. “Jesus, I never realised how little I knew about the rest of the world until I came here.”