Taharos, Chapter 9, Mishnah 6. Our chapter follows olives from the moment they are picked until they reach the press, and it keeps returning to one question: at what point does the liquid seeping out of a pile of olives render them susceptible to tumah? The rule behind every case is that liquid only creates hechsher, susceptibility, when the owner is pleased to have it there. Moisture that a person wants serves him; moisture that simply happens to him, against his wishes, leaves the fruit exactly as insusceptible as it was before. This mishnah, and the one that follows, work out that principle in a series of everyday situations.
The opening case: "Hamaniach zeisim bagag legargeran, afilu hen rum amah, einan muchsharin." Someone lays out his olives up on the roof so the sun will dry them into gargerim, shriveled olives. Even when the heap rises a full amah, they have not been rendered muchsharin. Picture such a mound: as olives shrivel they release their sap, and the higher the pile, the more of that juice pools underneath, so liquid is certainly present. Yet it accomplishes nothing here. A man who sets his crop out under the sun is after dry fruit, and the very wetness that appears is what he means to lose. Liquid that comes against the owner's will has no bearing at all on whether the fruit can take on tumah.
Now the mishnah turns the case around: "Netanan babayis sheyeilku ve'asid leha'alosan lagag, o shenesanan bagag sheyeilku o sheyiftechu, harei eilu muchsharin." He put them in the house so that they should soften and wither, with the intention of bringing them up to the roof afterward; or he put them on the roof itself so that they should wither there, or so that they should split open, since split olives are what he needs in order to salt them. In all of these, the olives are muchsharin. Here the softening is his goal, the seeping liquid is part of the process he set in motion, and moisture that a person wants is moisture that makes food susceptible.
The final case looks similar on the surface but comes out the other way: "Netanan babayis ad sheyishmor es gago, o ad sheyolichem lemakom acheir, einan muchsharin." He put the olives in the house only until he could put his roof in order. Perhaps he had not yet built the ma'akeh, the parapet, and he may not bring his crop up there until the roof is fit; or he is holding them in the house only until he has a chance to carry them off somewhere else. In such a case they are not muchsharin. His plan lies elsewhere, on the roof or at another location, and the stay in the house is nothing but a delay. Whatever liquid gathers during that interval is undesired, and it does not touch the susceptibility of the olives at all.
Three cases, one measure. Not the amount of liquid, not the size of the heap, not where the olives happen to be lying, but only this: is the moisture what the owner wanted, or is it something he is waiting out?