You might be under the impression that young blood is the fountain of youth. A number of studies conducted in older mice (and, once, humans) over the past 10 years have shown benefits from transfusions of young blood, or parabiosis, restoring cognitive function or regenerating muscles. Yesterday, another study was published, in which older mice with broken bones healed faster when injected with young blood. But the mechanism behind these supposed benefits is still being debated, so don't expect to be able to ask your doctor for a quick transfusion of young blood anytime soon.