Tubby was on her way to a talk on number theory when a man photographed her in the lobby. He looked a bit younger than her son, which meant he was probably about the same age, only athletic and well dressed. She looked at him a moment; he waved and said something in English. She grunted and began to hustle away, but then he repeated it in Mandarin: "Excuse me, can I ask your name?"

"You can call me Teacher Shu."

"Thanks." He fiddled with the phone he'd used to take the picture.

"Are you writing it down?" She turned her most baleful teacher's gaze on him. "Is this some kind of scam?"

"No, Teacher, no scam," he said, and his smile was bright with pain. "I have to do it."

Tubby noticed his badge. It had a barcode and a photo; visitors' badges just had numbers. He saw her see it. "That's right, Teacher," he said. "I'm your new neighbor." He turned back to his phone.

Tubby walked up to him, the sternness of her regard undiminished. "It's polite to introduce yourself to a neighbor."

"Sorry," he said, not looking up. "Jim Chen. It's nice to meet you, Teacher Shu." He looked up and pointed at a lamp on an end table. "Can you stand over there and make a face?"

"What is this about?"

"Stand by the lamp and I'll tell you."

Tubby grimaced. Mr. Chen caught the expression in a snapshot. "Good enough. Three, two, one... crap." He showed Tubby the phone. On the screen was a photo of a balding man a few years older than Tubby, dressed in a white lab coat, with a sour expression on his face. There was an orange square in the corner of the photo, and a bunch of English under it. "It thought you were Dr. Hanrahan," said Mr. Chen. "Neurologist at Plainsboro, questionably trustworthy."

"How do you know whether he's trustworthy?"

Mr. Chen pointed to the orange square. "I flagged him. There should be some notes in his file that explain it. Anyway, the face recognition doesn't work yet, but you're in my library now, Teacher Shu. Every photo helps." He pulled up the new photo of her, which was horrible, and started fiddling again.

Tubby looked Mr. Chen up and down, as though she might find a physical clue to his affliction. "What happened to you?"

Mr. Chen shrugged. "My brain quit making new memories. Luckily, the problem didn't spread to my phone." That thought made him pause for a moment, frowning. "Of course, if it had, I'd never know."

Tubby grunted. "Good-bye, Mr. Chen."

He had already returned to his phone. Tubby walked away without another word, content not to extract the pleasantry he owed her. There was still time to catch the train.