When an adult loses a tooth, there's no hope of growing a new one—unless you've got a mouse kidney handy. In a new study, researchers injected human gum tissue extracted during oral surgery into the molars of fetal mice. After giving the cells a week to get used to each other, the scientists implanted the chimeric concoction into the protective tissue surrounding the kidneys of living mice. There, 20% of the cells developed into objects recognizable as teeth (more real than the dentures pictured above), complete with the root structures missing from artificial tooth implants. The next step is to transplant these so-called "bioteeth" back into human mouths and see if they grow into something that we can chew on—or rather, with.