One major change is that unused connections in the thinking and processing part of the adolescent brain (the grey matter) are 'pruned' away. This is the brains way of becoming more efficient, based on the 'use it or lose it' principle. Synaptic pruning is thought to help the brain transition from childhood, when it is able to learn and make new connections easily, to adulthood, when it is a bit more settled in structure, but can focus on a single problem for longer and carry out more complex thought processes. The pruning process begins at the back of the brain. The front part of the brain, the pre frontal cortex, is developed last (latter adolescence/early adulthood.