This lecture covers the (s,s)-inventory model, focusing on the inventory system's level at the end of the period, demand, and order policy. It also discusses the concept of stationary Markov chains, irreducible chains, and the invariant distribution. The instructor explains the intuition behind the time reversal property in Markov chains and the detailed balance condition. Additionally, a queueing example and the Ehrenfest urns model are presented, illustrating the thermodynamics of gas molecules. The lecture concludes with the discussion of the invariant distribution in the context of detailed balance conditions and the binomial distribution.