Four years ago, Audie Leventhal's daughters sat him down and explained to him, slowly, that they were smarter than he was. Typical teenage behavior, but Leventhal took them seriously. "It wasn't that I thought they were smarter," he says, laughing. "But I wondered, was I getting stupider?" Well, maybe. Leventhal, a neurobiologist, has just published research that leaves no doubt as to his intellect--but it does indicate that we're all, indeed, getting stupider with age. And for the first time it hints at why.