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{{Thermodynamics|cTopic=[[List of thermodynamic properties|System properties]]}}
In [[thermodynamics]], entropy is a numerical quantity that shows that many physical processes can go in only one direction in time. For example, cream and coffee can be mixed together, but cannot be "unmixed"; a piece of wood can be burned, but cannot be "unburned". The word 'entropy' has entered popular usage to refer to a lack of order or predictability, or of a gradual decline into disorder.<ref name="lexico">{{cite web |title=Definition of entropy in English |url=https://www.lexico.com/en/definition/entropy |archive-url=https://web.archive.org/web/20190711005908/https://www.lexico.com/en/definition/entropy |url-status=dead |archive-date=July 11, 2019 |website=Lexico Powered By Oxford |access-date=18 November 2020}}</ref> A more physical interpretation of thermodynamic entropy refers to spread of energy or matter, or to extent and diversity of microscopic motion.
If a movie that shows coffee being mixed or wood being burned is played in reverse, it would depict processes highly improbable in reality. Mixing coffee and burning wood are "irreversible". Irreversibility is described by a law of nature known as the [[second law of thermodynamics]], which states that in an isolated system (a system not connected to any other system) which is undergoing change, entropy increases over time.<ref>Theoretically, coffee can be "unmixed" and wood can be "unburned", but this would need a "machine" that would generate more entropy than was lost in the original process. This is why the second law only holds for isolated system which means they cannot be connected to some external "machine".</ref>
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