 That fact is already on its own remarkable: all other known fundamental principles and laws of physics are time-symmetric (putting aside the minuscule violations of time-reversal symmetry manifested by the weak nuclear force). That means that if a given sequence of changes of a physical system governed by those principles and laws is allowed, then the same sequence in reversed order is also allowed as a physical possibility. If a tea cup drops to the floor and smashes into little bits, then the reverse process is also possible: the smashed bits can spontaneously leap up into the air towards each other and re-assemble into an undamaged tea cup. If an antenna can absorb a given type of radio wave, it can also emit that same wave. And so on. That is not what we see in physical systems governed by thermodynamics and the Second Law. An ice cube in a glass of warm water spontaneously melts, and the water cools a bit. We never see a cool glass of water spontaneously warm while an ice cube forms in the middle of it. This is even more mysterious when one considers the fact that we know that the water and ice are, at a deeper level of description, really just a collection of an enormous number of molecules and atoms themselves moving around, bouncing off each other, and connected together—and, to the best of our knowledge, the principles and laws governing the changes in that collection of molecules are time symmetric. Why is it that the laws governing the micro-structure of water and ice are time symmetric, but, when one looks at the water and ice in the aggregate, ignoring the fine details of the micro-structure, the governing principle becomes time asymmetric? That is one of the deepest and most hotly debated questions in the foundations of physics. 