What happens when we think? Why are some brains resistant to decline? How do you make a brain? What makes you smart? Find out in our guide to the most important organ

Liam Madden

Inside your head is an object capable of feats of computation, creativity and understanding unrivalled in the known universe – and all using the power of a 20-watt light bulb.

We have made huge strides in understanding the human brain. In recent years, we have discovered that brain cells can regenerate and pinned down what happens when you start talking before you know what you want to say. Yet, the more we learn, the more we realise how much we still don’t know. In the following pages, we explore the biggest questions about the brain to reveal the mechanisms and mysteries of this phenomenal blob of grey goo.

What makes our brain special?

The human brain, we love to tell ourselves, is exceptional. Other animals might use tools or solve mazes, but can they invent computers or write sonnets?

Yet even with our extraordinary mental prowess, it isn’t easy to explain what makes the human brain so special. At around 1.5 kilograms, our brains are about a third the weight of an elephant’s and a fifth that of a sperm whale.

If body size is taken into account, however, our brains are unusually large: between seven and eight times what would be expected for a mammal our size. But this crude measure isn’t enough to explain our intelligence. The brain-to-body-size ratio of a capuchin monkey is higher than that of a gorilla, yet gorillas are considered smarter.

Clearly size isn’t everything. A more important metric might be the number of neurons – the brain’s processing units. Humans have about 86 billion, according to Suzana Herculano-Houzel of Vanderbilt University …