Your brain is a very energy demanding organ: it only accounts for 2% of your body weight but consumes about 20% of your daily energy. All of this energy comes from a continual supply of metabolic fuel, which includes glucose but also other energy-rich molecules, and the details turn out to be important for brain function. Many people who experience epileptic seizures do not respond to any medications, but they can be treated with a change in diet that produces a change in the fuel mix used by the brain. Dr. Yellen discusses our understanding of this and other puzzles in brain metabolism.