Can tiny brains grown in a dish reveal the secrets of sociability? Balls of brain tissue generated from stem cells are enabling us to understand the underlying differences between people who struggle to be sociable and those who have difficulty reining themselves in. Alysson Muotri at the University of California, San Diego, and his team created the mini-brains by exposing stem cells taken from the pulp of children’s milk teeth to cocktails of growth factors that help them mature.