Exercise yields more mitochondria in the brain?

Researchers have long known that exercise is good for more than just muscular and cardiovascular health. It also can make a person's brain function better. Using mice as subjects they followed this hypothesis. 

Some questions to ponder as you comment: Why are the mitochondria so important in this research? How can the tell the mitochondrial DNA and nuclear DNA apart? How is this an example of homeostasis? How could this information be used to improve school performance?