Entropy; the descent into chaos aka the way that nature intended. 'We are compelled to travel into the future and that is because the arrow of time dictates that as each moment passes, things change and once these changes have happened they are never undone. Permanent change is a fundamental part of what it means to be human and it is fundamental in the universe. No matter what, everything is always, irreversibly changing. This can be explained through the second law of thermodynamics, which states basically that in a closed system, entropy will always increase; entropy being the quantative measure of disorder within a system. Entropy explains why when left to the elements, mortar crumbles, glass shatters and buildings collapse. To understand this law you must think of objects not as single things but as being made up of many constituent parts... Everything will always follow a set path from low entropy structures to high entropy structures, from array towards disarray, from order to chaos.' (Paraphrased from) Brian Cox, The Wonders of the Universe, BBC.