As far as we know, there’s only one thing in our solar system sophisticated enough to study itself: the human brain. But this self-investigation is challenging because a living brain is shielded by skull, swaddled in tissue, and made up of billions of tiny cells. How do we study living brains without harming their owners? Elizabeth Waters and John Brorghi explain how EEGs, fMRIs, and PETs work. Lesson by Elizabeth Waters and John Borghi, animation by Joash Berkeley.