You are not the person you used to be. Two weeks ago, the surface of your skin was covered with a completely different set of cells, which have since died and flaked off. Four month ago, you had a wholly different set of red blood cells. Since birth, your body has grown tremendously in size and much of it is constantly regenerating, replacing old cells with new ones.

But your brain is different. At birth, the part of your brain that controls your most human abilities – the neocortex – came fully equipped with 100 billion neurons. These same neurons have lasted throughout the years and still power your thoughts today.

The neocortex makes up most of the brain. Its relatively