Oh, to be a mouse. You'd have been cured of every conceivable cancer and now, you'd have something of a fountain of youth, too, at least for your thymus. A team of researchers has gotten immune-system organs called thymuses in mice to grow, The Economist reports. After the researchers' treatment, the mice's thymuses grew more than twice as large. The thymuses, which normally change in structure and become less efficient as animals age, also returned to a more youthful structure.