The brain is very, very complex. There are 100,000,000,000 neurons, with each one being connected to 10,000 others, and all the connections can be strengthened and weakened. The pattern of connections matters – it is what makes us feel emotion, think, see, hear, understand, remember and so on. We don’t really understand how it works and this is what makes it so interesting compared with other organs.
Everything that makes our existence seem meaningful comes from the brain. So much so, that if someone still has a heathy heart and lungs, but there brain is lifeless, we call them dead (this happens with modern life-support systems that can keep the heart and lungs going forever).