A teenager's brain is still only 90-95% developed. During adolescence, the brain strengthens the connections. This is the brain's way of becoming more efficient. Because the prefrontal cortex is still developing, teenagers might rely on a part of the brain called the amygdala to make decisions and solve problems more than adults do. The amygdala is associated with emotions, impulses, aggression and instinctive behavior. These changes are significant because they are crucial to how a teen transfers into adulthood.