In a series of papers released on Sunday, researchers showed that it’s possible to reduce many of the problems with age, including declines in memory and loss of muscle strength, simply by supplying older mice with blood plasma from younger ones. The work even identified a specific protein, GDF11, that mediates many of the effects. The work suggests that some aspects of the aging brain aren’t necessarily an internal feature of the neurons that comprise it, but a product of their interactions with their environment—an environment that could potentially be changed. The work grew out of a rather unusual laboratory technique that creates what are termed “parabionts,” genetically matched animals in which patches of skin are surgically fused. Eventually, their circulatory systems mesh, allowing factors from one animal to freely move through the body of the other.