Jeanne Calment said she was 122, but there were questions. The records from 1875 were shaky, some of them deliberately burned. Tanaka Kane, 119, was on firmer ground, and then there were loads of others in the hundred-teens. For some time 120 seemed to be a firm limit.

Belinda didn’t think much about it as her great-grandmother grew older. Granny Lindsey had always been a firecracker, but more to the point she’d always been there—starting another business after she retired and passed the first one off to Belinda’s uncle. “Instagran,” she called herself, a joke Belinda had never gotten, old people humor about some website that had been cutting edge before Belinda’s mother was born, but she loved Granny anyway. She’d marveled at the way she picked up new things that even younger family members struggled to understand.

She dimly remembered a time when her grandfather and aunts and mother were all making worried noises about whether Granny’s memory was going, when Belinda was in her teens and laser-focused on herself. By the time she had enough perspective to think past her own arm, no one seemed worried, and certainly Belinda saw no cause for worry herself.

Granny’s friends seemed to be dying off at an alarming rate, but she was quite good at making more friends, always had been. So while this was sad, it didn’t seem to be an insurmountable problem. Every year there always seemed to be new people with new ideas to invite to share the cake with cream and strawberries, or every five years the big family gathering with a banner. From time to time one of the aunts said something about how the old girl wouldn’t be with us forever. Aunts said that. It was in the aunting contract. Any moment now one of Belinda’s cousins would have a baby and she’d start saying it herself.

It was a blow when her grandfather died. Belinda had never lost someone close to her before. All her friends made noises about how he’d had a long, full life, but that didn’t make any sense when his mother was right there. And yet there he was, ninety, clearly an old man. She looked around her at a world where her Granny was not just old but the mother of an old person.

Really really old.

And her college roommate pinged her implants with a story reading, “One! Two! Three! One hundred twenty-three-year-old woman breaks records, still going strong!”

The woman was only two years and a month older than Granny Lindsey.

Belinda was helping her great-grandmother spring clean the kitchen when the call came in. Granny put it on speaker so they both could hear. “My great-granddaughter is here with me,” she explained. “I read your prospectus, but I don’t have it in front of me. Belinda, this doctor wants to talk about longevity.”

“I think you’re full up on that, Granny,” said Belinda, laughing nervously.

“As your joke is acknowledging, that’s why I’ve called. I’m Higashi Chiyo, and I’m team lead for a research group studying the outer limits of human longevity. There’s a cohort of people like your great-grandmother, Belinda, who are challenging what we thought we knew about aging.”

“Oh, she will challenge you.” Belinda tried to put the serving dishes back on the high shelf without making noise that the noise-cancellation program would have trouble filtering out.

“We want to run some tests. Nothing too invasive, mostly questions, a lot of scans. A little blood work. We want to find out why Ms. Nilsson is—”

“Still alive,” Granny filled in cheerfully.

“Well, yes.”

“I don’t see any reason why not.”

“Oh good! I’ll send you the information.”

When the call was disconnected, Granny confessed, “I actually meant that I didn’t see any reason not to carry on being alive, but she seems nice, I don’t want to disappoint her. If they want me to do anything too nasty I’ll just say no.”

“Do you want me to go with you?” Belinda asked.

“Oh, that’d be nice, dear.”

Belinda wasn’t sure what she’d expected of a gerontology and senescence lab, but it wasn’t the pleasant, deep colors and soft furniture that greeted her. She was ushered into a waiting room with other friends and relations of varying ages and degrees of put out that they were not central to the proceedings. She shrugged and pulled up a book on her implant to “read” while she waited. She got through most of a chapter before the voice she recognized from her granny’s speaker call presented itself as a petite, tidy woman beckoning her to come look through glass at old people doing tests.

“It’s an exciting project, we really don’t know where it’s going to go,” said Dr. Higashi.

“We sort of . . . took it for granted that she’d keep going, I’m afraid,” said Belinda. “And she has. But you think you know why?”

“That’s what we’re trying to figure out. In the periodic table, we talk about islands of stability,” said Dr. Higashi. “Sometimes they’re called doubly magic numbers. Places where the configuration of protons and neutrons can find stability again even though there are more of them.”

“My granny is plutonium,” said Belinda.

“No, no, quite the opposite!” said Dr. Higashi. “Plutonium—forgive me for the unscientific phrasing—flies apart. Goes kaboom. Your grandmother just sort of . . . hangs together. Body and brain, she just keeps going.”

“And aren’t we glad,” Belinda agreed, watching through the window as Granny Lindsey took another reaction test. Was she getting faster at them? Practice made perfect, but no one ever claimed it was supposed to make more perfect indefinitely.

“The thing is,” Dr. Higashi continued, “we know some of the early bumps that people get through to live longer. If an allergic reaction doesn’t kill you at this age, or childbirth at that age. We used to think cancer was the sort of thing that would get you if nothing else did. But now we’re wondering whether you can actually live past your cancer age. And then what gets you?”

Belinda waited to see whether there were more details about this forthcoming, but Dr. Higashi was intent on watching Granny’s tests. As they progressed, Belinda lost the context to see why: the tests proceeded into areas that were less and less obvious to the untrained eye.

But Granny seemed to be more and more fascinated with them. Her eyes sparkled, her wrinkly little chin went up, and she was more engaged than she had been in ages. Belinda had always assumed her great-grandmother was happy. She laughed at jokes, she enjoyed cakes, she had her strange little businesses that other old people liked. What was there to expect of being 123 years old?

“Oh, look at that,” breathed Dr. Higashi.

“At what?” asked Belinda.

“She’s finished faster than the other hundred-twenty-somethings.”

“Good?”

“Perhaps.”

They met in a room where Dr. Higashi talked about the idea of islands of stability, of how far a human might go. She talked of liver enzymes, of neural pathways. She talked of telomere stability beyond lifestyle choice.

“It sounds like I have quite a lot to do here,” said Granny. “I think, if you don’t mind, the liver sounds like the first thing I’ll focus on.”

“I’m so glad you want to be one of our subjects,” said Dr. Higashi. But Belinda knew that light, though she’d never seen it before.

“Oh no, darlings,” said Granny gently. “We’re not the subjects. Or rather, we are. But we’re also the researchers. After all, you don’t last this long without being quite good at finding something to latch onto. And this? This will do quite nicely.”