Every biological process requires energy. Mitochondria, the parts of a cell that break down energy, have a breathtaking vitality. These dynamic bodies – up to 1,000 in each cell – zoom around, continuously changing shape, dividing into segments and then bonding back together. Trillions of mitochondria perform their dances, nonstop, during a person’s lifetime. The need to understand this dance, and the other intricacies of human metabolism, is critical to better medical treatment.