Like young blood, young poop is making headlines for its potential to reverse age-related decline in mice. Now, a new study has flipped the script: The poop of older donors has rejuvenated the ovaries of younger mice. At first, researchers at the University of Southern California, Los Angeles (USC) thought that if they transplanted the gut microbes from 'menopausal' mice into younger mice whose microbiomes had been completely wiped out, it would age their ovaries. Much to their surprise,