&gt;If you mix oil and water, you will see the result does unmix itself again. Does this mean entropy decreased and time direction got reversed? Nah.<p>Moving an object to higher gravitational potential decreases the entropy of the system (and thus requires work, unsurprisingly).<p>Mixing oil into water means you displace some volume of  water which were closer to Earth with oil bubbles (which are lighter than equivalent volume of water) while you force the displaced water volume higher&#x2F;further from Earth. Thus by mixing lighter oil with heavier water you decrease the entropy of the gravitational system consisting of the oil, water and Earth. Left to its own devices, the system evolves along the entropy increasing gradient - heavy water goes down displacing lighter oil up, thus decreasing the gravitational potential of the system (the friction during that &quot;unmixing&quot; is the work of the gravitational force that ultimately results in the heat that is lost away into the  environment).