When you start something new, a large number of neurons in the frontal cortex (the higher, more conscious command area of the brain) are recruited and become active, helping you in the learning process.

FORBES: Why Apprenticeships Are the Fastest Way to Acquire Skills

Command: Harrell is commander-in-chief of Special Operations Command Central, which is responsible for planning special operations throughout U.S. Central Command's area of responsibility.

CNN: Specials

Petraeus, who took over Friday as head of U.S. Central Command, is overseeing his own review across the 20 countries that make up the command's area of operations in the Middle East and Asia.

CNN: Analysis: U.S. reviewing Afghanistan policy