Scientists may one day slow down aging with a simple injection of youthful stem cells. They’ve just proven this can be done in mice, according to a study published Tuesday in Nature Communications. The mice, which had been engineered to mimic a human disease called progeria, would normally have grown old when they were quite young. But that changed when researchers injected muscle stem cells from healthy young mice into the bellies of the quickly aging mice. Within days, the doddering and frail mice began to act like they were living the storyline of “The Strange Case of Benjamin Button” as they started looking and acting younger.