It has nothing to do with the immaculate execution of the work, although this really is the most beautiful crack I have seen. Nor its realism - though the work even has its own splinter-cracks, which fortunately peter out before they reach the Tate bookshop. Nor its popular success - although last Saturday I admired the hordes following the path of the crevice, and the huddle of children at the far end trying to stick their hands under a wall of frosted glass, searching for the end of the illusion.