Around 1.4 kilograms, and about 86 billion neurons. It is two per cent of your body weight and takes roughly twenty per cent of your energy, whether you are solving something or staring out of a window. The folding is not decoration: unfolded, the cortex is a sheet of roughly 1,800 to 2,400 square centimetres, about the area of a pillowcase, and it has to fit in a skull the size of a grapefruit. Two thirds of it is buried in the creases — the sulci — and only the crowns, the gyri, are on the surface you can see.