An accidentally discovered object is described. Close contact with it convinces that it is the very object which has been taken in due time for an expansion shock and replaced by an automodel Riemann construction. It is shown that this object is not shock in its usual self. So there is no any cause to break with it for reasons of entropy nondecreasing.
A traditional treatment of thermodynamics principle in today's continuum mechanics is adapted artificially to substantiating physical nature of Riemann model for an expansion wave. Actually this substantiation rests on physical narrowness of differential form of gas-dynamic equations for which this wave serves as a limiting solution. The matter is that in derivation of these equations one uses implicitly the principle of long range action which appears to be decisive in settling this vexed question.
The paper is debatable. It is accompanied by the opinion of the reader Yu.A. Bondarenko contrary to the author's concepts.
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