Jay Giedd, an NIMH neuroscientist who pioneered the early MRI research on teen brains, is fond of saying that "what's important is the journey." Researchers caution that they can't prove links between brain parts and behavior, or that tackling adult-size challenges will turn teenagers into better adults. But common sense suggests that Nature had a reason to give adolescents strong bodies, impulsive natures, and curious, flexible minds. "Our generation is ready for more," insists Alex Harris, 20, of Gresham, Ore., who, with his twin brother, Brett, writes a blog and has published a book urging teens to push themselves. Its title: "Do Hard Things."