It's called neurogenesis. Your brain's hippocampus is capable of creating new neurons and certain environmental factors make it speed up and slow down.
The creation of new brain cells, once thought impossible, is related to memory. Things like eating healthy food and exercising can enhance your body's ability to create new ones while stress, high levels of sugar and other negatives can slow it down.
Here's a cross-section; you can see where the new neurons incubate
This is one of those "under the radar" pieces of information that has made its way into the main-stream. You are constantly regrowing and regenerating your body.
Doctor Sandrine Thuret, neural stem cell researcher at the King's College of London, has examined the power and potential of the human body to grow new brain cells. Her research has uncovered a slew of activities that are good for regrowth. She has also isolated some very dangerous behavior that will slow down the production of new brain cells, including excessive sugar, stress and lack of sleep.