The most common illustration of entropy is the box with the partition and the two gases on either side. It took energy to separate the gases on either side of the partition, but if we remove the partition, no additional work needs to be done in order to get them to mix. We must expend more energy if we want to separate the gases again. So it's concluded that entropy is always on the rise, since no work needs to be done in order to sustain it. But what about oil and water. In fact, we don't even have to bring up oil and water. Let's consider the gases. Which gases are they? Helium and argon perhaps? Both are noble so they won't interact, and one has a higher atomic mass than the other, so one will settle in the middle of the box (assuming the box is in free fall) while the other rests beyond it. The gases will naturally separate themselves, just as the iron of a molten planet sinks to the bottom and the water floats to the top. Like oil and water, many admixtures of elements and compounds naturally tend toward heterogeneity. The longer a rock has to cool, the larger its constituent crystals are. Anyways, my question is "Why do we really believe that entropy is on the rise?"