A toy model is used to describe the following steps to achieve the no-reversible-direction axiom in a tutorial manner: (i) choose a state space results in the balance equations on state space which are linear in the process directions, (ii) avoid a reversible process direction that cannot be generated via a combination of non-reversible ones, (iii) process directions that are in the kernel of the balance equations and do not enter the entropy production. The Coleman-Mizel formulation of the second law and the Liu relations follow immediately.
Keywords: Liu relations; balance and constitutive equations; dissipation inequality; no-reversible-direction axiom; process directions; second law.