London cabbies' brains are like muscles - with the right exercise, they can grow. Neuroscientists have found that all those hours London cabbies spend memorizing the city streets causes the spatial memory region of their brains to get bigger and physically larger. But there's a catch. To acquire the knowledge, one part of his brain had to grow, but that caused another region to shrink. And that shrinkage comes at a cost. It means our powers of recall in other areas aren't nearly as strong.