The tendency toward equilibrium is so fundamental to physics that the second law is probably the most universal regulator of natural activity known to science. The concept of temperature enters into thermodynamics as a precise mathematical quantity that relates heat to entropy. The interplay of these three quantities is further constrained by the third law of thermodynamics, which deals with the absolute zero of temperature and its theoretical unattainability. The third law states that, as this minimum temperature is approached, the further extraction of energy becomes more and more difficult. All the fundamental Newtonian laws are time reversible.
Collisions look the same forwards or backwards.
A box of gas molecules obeying Newton's laws perfectly does not have an inbuilt arrow of time. However, it is possible to show that the continual random molecular motions will cause the entire ensemble to visit and revisit every possible state of the box, much like the continual shuffling of a deck of cards will eventually reproduce any sequence. This ability of Nature to be divided into a multitude of states makes it easier to understand why thermodynamical systems move toward equilibrium, known as Poincare's theorem. If a box of gas is in a low entropy state at one moment, it will very probably soon be in a less ordered state since given the large number of states for it to evolve to, most of those states are of higher entropy. So just by the laws of chance, the box has a higher probability of becoming a higher entropy state rather than a lower one since there are so many more possible high entropy states.