While Newton's laws do not forbid all the hot balls to gather on one side and all the cool ones on the other side, the probability that this will happen is practically zero. There are a very large number of ways to distribute the energy among all the balls. Any one specific way is equally unlikely . But there are many more specific ways of distributing the energy so that the average energy is approximately equal on both sides than there are of distributing the energy so that the average kinetic energy is tree times as high on side 2 as it is on side 1. There are many more ways of having a disorderly arrangement than of having an orderly arrangement.
Disorder is more likely than order.
The amount of disorder is the number of ways the insides of a system can be arranged so that from the outside things looks the same. It turns out that the logarithm of that number of ways is proportional the entropy. We can define the entropy as the logarithm of the disorder times some constant of proportionality . Entropy always increases, because a high amount of disorder is, by definition, is more likely than a low amount of disorder. With our definition of disorder, every condition of a system has a well-defined disorder.