One of the greatest challenges of the 21st century is understanding the brain. The most complex organ in the human body, it is the brain that produces our every thought, action, memory, feeling and experience of the world. The brain is estimated to contain about 80 to 120 billion neurons, and these make contact with thousands or even tens of thousands of others via synapses. Moheb Costandi is a molecular and developmental neurobiologist, and a science writer for the Guardian and Scientific American. In 50 Human Brain Ideas You Really Need to Know, there are 50 short chapters, each devoted to a concept that is fundamental to our current understanding of the brain.