"The human brain can readily change its structure-- a phenomenon scientists call neuroplasticity," writes Jim Holt in the New York Times.  "A few years ago, brain scans of London cabbies showed that the detailed mental maps they had built up in the course of navigating their city's complicated streets were apparent in their brains.  Not only was the posterior hippocampus-- one area of the brain where spatial representations are stored-- larger in the drivers; the increase in size was proportional to the number of years they had on the job."

These cabbies of the urban jungle are living proof that exercising your brain strengthens and conditions your mind.