Gray matter, the meaty, wrinkly stuff we often think of when we conjure images of the brain, is really only half the story when it comes to understanding what's going on up there. Recent research from the University of Illinois at Urbana-Champaign examining both children and older adults shows that the brain's white matter—the network of nerve fibers that connect and transmit brain signals from one part of the brain to another—becomes more fibrous and compact with physical exercise.