When I think about my head balancing on top of my spine it reminds me a little of these zen rocks - a beautiful arrangement. The bones of the human skeleton are stacked up high, rather like these rocks, but of course we can move about, on relatively tiny little feet, in a continuous, incredible relationship with gravity. The key to upright balance is the interaction of gravity with the human head - which is astonishingly heavy - approximately 11lb or 5k. I have a bag of sand