The experimental drug J147 is something of a modern elixir of life; it’s been shown to treat Alzheimer’s disease and reverse aging in mice and is almost ready for clinical trials in humans. Now scientists have solved the puzzle of what, exactly, J147 does. They report that the drug binds to a protein found in mitochondria, the energy-generating powerhouses of cells. In turn, they showed, it makes aging cells, mice and flies appear more youthful.