Every head is a world.
The physical base of your mind is of course your brain, the grey matter housed in your head. And what a wonder it is! Your brain is composed of about 10,000 million cells. In fact, it has more cells than there are people on the face of the earth! Each one of those cells can link up with approximately 10,000 of its neighbours, which gives you some 10,000 billion possible combinations.
Amazing, isn't it? But there's more: