Disorder increases irreversibly. This statement does not necessarily apply at any given time to a child’s bedroom or to the way the world works. Rather, it is the statement of the second principle of thermodynamics, expressed by the physicist Sadi Carnot in 1824. It is a principle that can be experienced every day. When milk is poured into water, for example, the two liquids mix and do not remain separated from each other. Playing balls in a bag will not spontaneously line up according to their colour but will mix randomly. While it is easy to mix two gases together, it is almost impossible to separate them once they have been brought together. This talk takes a look at a simple mathematical model that explains why we can observe spontaneous mixing but not the opposite phenomenon. Spoiler alert: the key to understanding this temporal irreversibility lies in probability theory and more precisely in the law of large numbers.