Transfusing young mice with blood from older rodents quickly triggers ageing, suggesting that cellular ageing isn't just a case of wear and tear. There is a longstanding hypothesis that surgically connecting an old mouse with a young rodent causes a transfer of blood that de-ages the older animal. While this benefits the older mouse, the effects on the young donor rodent were less clear. Irina Conboy at the University of California, Berkeley , and her colleagues transfused blood between young and old mice. Those aged 3 months got blood from animals that were approaching 2 years old. Two weeks later, the young mice had an increased number of senescent cells ? cells in the liver, kidneys and muscles that are damaged and stop dividing, but don't die. Strength tests also revealed the young mice became weaker after receiving the older rodents' blood. "Cell senescence is only part of the process of ageing," says Conboy. "That opens new horizons and helps explain why senolytics (drugs that clear senescent cells in the body) so far in clinical trials were less successful than people hoped."