A collection of rembrandts self-portraits serve as an illustration of the passage of time

When left to itself snow spontaneously would never build a snowman.
It will only form different kinds of heaps . This can be undestood as the consequence of statistics: There are more possible arrangements of  the atoms constituting the snow leading to unordered heaps than arrangements resulting in the much more ordered state of a snowman,.

This cicumstance can be described with the concept of statistical weight.

The statistical weight (is the number of microscopic states(ways of arranging the atoms) %omega;corresponding to a certain macroscopic state(the large scale object).
The entropy, S, is defined as
S = k ln Ω
It is a measure of the disorder of the system. In all spontaneous processes the entropy increases because statistically the state with the greatest statistical Weight is more probable.
Such a process is irreversible.

In an adiabatic process no Heath is exchanged with The surroundings.