It's a commonly held belief that we lose brain cells as we age. But, in fact, although our brains may not work as well when we get older -- learning becomes harder, memories fuzzier -- the number of cells they contain remains the same, about 28 billion. Scientists think the real problem is that the myelin sheaths around neurons wear down, like the tires on an old car, and consequently the brain can't move as fast.

A child's brain, on the other hand, is like a superabsorbant sponge. It will mop up anything -- rules not to cross the road, drink bleach or look at the sun through a telescope. At the same time, children will happily believe in things as dotty as Father Christmas and hellfire. Yet at birth, the brain is far from fully developed. Major changes occur in the brains of babies, changes that affect the function of the brain in future life.

For example, 6-month-old human babies can easily distinguish between individual humans and individual monkeys, but adults, while automatically able to recognize different humans, are almost completely unable to tell one monkey from another. At first sight this may seem like a wantonly bizarre fact, but it might help explain "perceptual narrowing." A good example of this phenomenon is our diminishing ability, as we grow older, to differentiate between different speech sounds if they don't occur in our mother tongue.